You'd think that alien species would be radically different - insectoids, three-legged wombats, giant cats, etc. Sometimes they're not even that far away. They look totally human and sound human. In some cases, this may well be a disguise or A Form You Are Comfortable With, but in others this appears to be their natural appearance.
One odd consequence of this, however, is that in the Federation Council scenes in the Star Trek movies, you often see very strange, non-humanoid (or only partly humanoid) aliens, because the movies have the necessary additional budget for them. These additional races are, of course, never seen in the TV series at all.
Gene Roddenberry gave more reasons for this in an interview once. Additionally, Roddenberry had always insisted that Star Trek was about human issues and that the aliens are intended as vehicles for social commentary. Budget constraints aside, if you try to make aliens look completely alien, you'll firstly make them look ridiculous (cf. Doctor Who), and secondly make it doubly hard for the actor playing the alien to do anything mildly resembling acting.
This has actually been isolated to extremely specific requirements: if an audience can't see an actor's eyes or mouth, their ability to empathize with or emotionally invest in that character is significantly impaired. If they can see neither, it's difficult to empathize with them at all. This is one reason why mooks, especially SF mooks like the Cylons or the Imperial Stormtroopers, are so often uniformed in face-obscuring helmets.
While not totally undoable with the post TOS Star Trek budget, the flamboyant and outlandish alien designs of Star Wars appeal to a more pulp Space Opera aesthetic from which Star Trek has historically chosen to distance itself, at least onscreen; the expanded universe of comics and novels is a different beast. This required aliens that may have been scientifically implausible (humanoid appearance, ability to communicate in English and emote like humans, etc) but easy for the human characters to interact with and the audience to relate to in the narrative.
Read also: Intriguing Cowbird Symbolism
The next step past Rubber Forehead Aliens (catlike or buglike or lizardlike aliens that can still sit in chairs and hold weapons) is Humanoid Aliens, possibly overlapping with Intelligent Gerbils. Contrast with Starfish Aliens. The Unintentional Uncanny Valley can result if your RF Alien looks a little too human. Possible sister trope to Bizarre Alien Limbs, if the make-up crew opts for weird rubber hands instead of facial appliances.
On a side note, Bill Blair holds the Guinness World Record for the most Rubber Forehead Aliens (202). His first science fiction makeup role was in Alien Nation and he never stopped. For starters, not only has he played in various Star Trek franchises as Cardassians, Jem'Hadar, Klingons, Borg and Vulcans (yes, all plural), but he has appeared in nearly every Babylon 5 episode as well as both movies.
His credit on the show? Cat Planet Cuties features Little Bit Beastly aliens like the Catians (cats) and Dogisians (dogs). Hilariously, the Catians originally called their home planet Earth and referred to themselves as Earthlings - they changed the name of their planet and species to Catia out of courtesy.
Dairugger XV: The aliens of Galveston are basically humans with blue skin and red eyes. Keats and Kirigas, the two Rugger team members from planet Mira, are a different shade of blue and have pointy ears. Digimon actually has this in the form of Agunimon, whose appearance is justified by him being the Human Spirit of Flame.
He looks mostly human, except that he has '80s Hair, fangs, and red markings on his face. The Saiyans in Dragon Ball Z are just humans with tails in appearance; that turn into giant monkeys during a full moon. Dragon Ball Z is very bad about this. There's even a whole race of "humans but skin is different color"; the Brench-seijin, which Jeice and Salza belong to.
Read also: Unveiling the Mystery of Space Aliens
Most of the major races fall into this. Most prominent besides the Saiyans are the Namekians, who are green-skinned and fanged with antenna and long pointy ears, but otherwise resemble human men, and the Shinjin, who look like humans with long pointy ears and weird skin colors (e.g. red, green, purple).
Practically all aliens in Leijiverse. Mazone, Illumidas, Tokarga... they're all humans with slightly different skin colour, even the Mazone who are plants. Miime's race is unrevealed, but she's also fully humanoid apart from not having visible mouth in most?
Macross has many examples, with the most notable being the Zentraedi; once you got past the whole size difference thing, they're nearly identical to humans, with the exception of skin and hair tones of varying offness to human norms. Later entries also tended to give pointy ears to Zentraedi (and humans with Zentraedi ancestry).
The Zolans in Macross 7 are another Protoculture-influenced species who appear mostly similar to humans, except for very Pointy Ears, two-toned brightly-colored hair, and prominent fur on the forearms of the men. The Ragnans of Macross Delta basically look like dark-skinned humans who happen to have gills, webbing, and fins.
Grey Alien
Read also: Pop Culture Aliens
There's also the Windermerians, who are physically indistinguishable from humans outside of having at least one antenna-like tentacle growing out of their hair, and the Voldorians, who are basically Little Bit Beastly Cat Folk. Negima! Magister Negi Magi does this with many inhabitants of the magic world; they look like normal people, but with horns or weird shaped ears or something.
UFO Robo Grendizer: The Vegans and other alien races have strangely-shaped heads or unusual skin or hair colours at the very least. Gandal has blue skin and his head looks cubic due to his very angular features, his wide forehead, and that the top of his head is nearly flat. Blackie has dark-purple skin, pointy ears and a very pointy, hairless head. One of the main characters in Ulysses 31 is Yumi, a young Zatrian girl.
Harry Vanderspeigle, the protagonist of Resident Alien. Lampshaded in some Space Agent Valérian book. Marvel Universe: The Kree are extraterrestrials with two main races: the blue-skinned (such as Ronan the Accuser), and others who seem completely human (such as Mar-Vell and Marvel Boy). There's a little problem: the blue Krees are dominant in the Kree social hierarchy, and treat the others as worthless slaves.
With such a background, Ronan's original opinion of us humans is nobody's surprise. Drang The Destroyer: The members of the interplanetary criminal syndicate the Circle of Evil are mostly human-looking with some distinctive inhuman trait like a lemon-yellow or chalk-white skin.
In Supergirl: Crucible, Lys Amata's unnamed alien species are pink-skinned, blue-haired humanoids with long oblong faces and black eyeballs. In Superman vs. "Superman vs. World's Finest (1941): In issue #176, Superman meets Dur, an alien from the Sirius solar system who looks like a human, if humans had scaly, chalk-white skin and large bulging black eyes. At the same time, Batman finds an alien called Tiron, who looks like an earless, blue-skinned, black-eyed human.
While The Worlds of Aldebaran features a huge variety of alien species, the only sapient aliens encountered so far look very much like humans, except that they are entirely hairless and have a flat nose reminiscent of a cat's muzzle (an impression reinforced by their yellow eyes and slit pupils). As in the source material, namely the Barsoom books, Martian civilizations in Warlord of Mars are indistinguishable from humans except for their unusual skin tone and laying eggs instead of giving birth.
The First-Born are grey skinned, the Okarans have golden-skin and the Red Martians are reddish-copper toned. The Psycogs and the Ethereals in Khaal: The Chronicles of a Galactic Emperor. Copperhead is home to a ridiculously diverse alien population. Tamaraneans such as Starfire of Teen Titans fame, are indistinguishable from humans, save the solid green eyes and gold/orange colored skin, yet are specifically stated to taxonomically be felines rather than apes.
Dan Dare: The Atlantines are human but with blue skin and a visible lump on the forehead. Lampshaded slightly in A Changed World, based on Star Trek Online. Dr. Warragul Wirrpanda, USS Bajor's chief medical officer and a human, tells a time-shifted Bajoran that "the only significant difference between you and a female of my species is some uterine quirks.
It's implied in the brief “real world” scene in The Keys Stand Alone: The Hard World that there is a hefty helping of Rubber-Forehead Aliens amongst the gamers. Mars Needs Moms: The Martians in this film are humanoid but have brownish or pinkish skin, oddly shaped heads, and dreadlock style hair. The Martians live in a female dominant gender segregated society underneath the surface of Mars.
The Evolution Of The Xenomorph (Animated)
In Galaxy Quest, the character of Dr. Lazarus from the Show Within the Show is played by Alexander Dane (who is in turn played by Alan Rickman) wearing a rubber forehead. The Thermians think he is a real alien, even though his rubber forehead begins to show damage and develop holes over the course of the adventure. The movie Trail Of The Screaming Forehead takes this to the logical extreme. The aliens are foreheads that attach themselves to humans.
The Fifth Element has a variety of particularly tacky examples. The alien opera-singer sort of looked like a hybrid between an Asari, a Twi'lek and a Xenomorph. Prince of Space: the men of Krankor are rubber nose aliens. Star Wars generally tries to avoid relying on this trope by including a number of Beast Man, Starfish Aliens and Humanoid Aliens that require full body costumes, puppetry, animatronics and later Serkis Folk to do.
However, there are quite a few species that are officially classified as "near-human". Mantis in Guardians of the Galaxy Vol. 2 looks like a human woman with two antennas sticking out of her forehead, an odd skin tone, and large black eyes. Most of the background races in the Guardians films qualify as well, usually having an inhuman skin tone and/or one or two prosthetics (e.g. The Men In Black films have a mix of this and Starfish Aliens.
Star Wars: The expanded universe has a lot of humanoid aliens. The Yuuzhan Vong from The New Jedi Order look like big, muscular humans with a few deliberate errors - their skin tones are varying shades of grey rather than brown, they have talons instead of fingernails, their foreheads are prominent and sloped, their eye-sockets droop like a basset hound's, and their hair is almost always black when they're not bald (which is more common among them than it is among humans).
Also they don't have kidneys. Artists also commonly depict them with pointed ears, though this is never described in the novels. InCryptid: Discussed by Sarah, who calls them "Star Trek aliens". The Lensman series has human, humanoid and utterly alien species. Justified in Last and First Men. The varieties of human aliens are a result of original humanity escaping from a dying Earth.
The axxa from S.F. Said's Phoenix have horns, hooves, and glowing eyes, but otherwise look human. Wicked Lovely tends to go this route with a lot of The Fair Folk. The aliens in Deathscent are somewhere between this and Humanoid Aliens. While both races look quite like humans - enough that when one first arrives the human characters don't realise what he is until they see him in the light - their biology is very different.
A large number of The Culture's alien species are near human. It was explained away as a convergent evolution thing. In fact, most of the books take place before humanity as we know it achieves spaceflight. Foreigner (1994) deals with the deceptively humanoid alien race known as the Atevi.
Subverted in the Uplift novel Brightness Reef. The Rothen, a human-like alien race trying to con human beings, wear artificial foreheads and other facial prosthetics to make themselves appear more human-like. The Tymbrimi are genuine Rubber Forehead Aliens, mostly-human looking though with different proportions and facial features, granted they're minor shapeshifters who can gradually change their features over time.
In Robert A. Heinlein's The Star Beast, the Rargyllian Dr. Ftaeml is described as a Medusiod Alien, being a humanoid with tentacles on his head resembling snakes. Older Than Feudalism: The very first "science fiction" novel, True History by the Ancient Roman author Lucian, has rubber forehead aliens living on the Moon... and the Sun and several stars.
A Moon-person looks basically human but has one toe on each foot, a marsupial pouch in his belly, a leaf growing out of his butt, and leaves for ears. The Catteni from Anne McCaffrey's Catteni quartet would be Human Aliens except for their grey skin. In Wasp (1957) by Eric Frank Russell, the Sirians are similar to humans, their most striking feature being their purple skin, a "bow-legged gait", and a number of other minor differences.
Zaphod Beeblebrox in The Hitchhiker's Guide to the Galaxy Trilogy could be considered one. He looks perfectly human except for his extra head. Xandri Corelel averts this for every species except the Anmerilli, who look almost exactly like humans except for their cheek ridges, bulbous foreheads, longer ears, and tails.
Crest of the Stars: The Abh are distinguished by their blue hair, though some of them also have pointy ears. This one does get justified, though, in that the Abh are in fact genetically altered humans, who even call their stellar nation the "Humankind Empire Abh" (or a variant, depending on how you translate it); the Abh see themselves as humans with a few different traits, while their (non-modded) enemies tend to see them as vile aliens, wholly different from humanity.
Space Brat: Surprising for a Coville book. Alien Nation. Babylon 5 is almost a case study on alien race-types on television. The Shadows and Vorlons are Starfish Aliens. Gang lord N'Grath is an insectoid species that was actually an animatronic puppet. Most other alien species in the 'verse - the Minbari, Narn, Drazi, etc. - are some version of this trope, or this mixed with Humanoid Aliens.
Buffy the Vampire Slayer and Angel have a lot of demonic races that are essentially humans in various different colors of facepaint. However, this is justified because almost all demons that appear are possessing or have interbred or been contaminated by humans. Thals are no different from humans other than bleach-blonde...
Grey-skinned (sometimes green-skinned) humanoids, usually 1 m (3.3 ft) tall, hairless, with large heads, black almond-shaped eyes, nostrils without a nose, slits for mouths, no ears and 3-4 fingers including thumb. Diminutive green humanoids. Even though a few abductions have referred to green skin, no report has ever involved anything that would fit the description of "little green men".
Cryptozoological animals, including those from folklore, religion (e.g. golem), mythology (e.g. Some claim that many of the allegedly real creatures from the Fortean archives (see also: Fortean Times and William R. Corliss) and related reports of anomalous phenomena[18] are actually of extraterrestrial or mixed origin, such as in the extraterrestrial hypothesis, the interdimensional hypothesis, or the cryptoterrestrial hypothesis.
Sometimes these creatures are associated with the occult or with esotericism, or linked with supernatural or paranormal phenomena. Others dismiss these explanations in favor of skepticism, cultural tracking, or the psychosocial hypothesis such as in cases of mass hysteria. Tall, scaly humanoids.
Reptilian humanoid beings date back at least as far as Ancient Egypt, with the crocodile-headed river god Sobek. Elongated visual artifacts appearing in photos and video recordings, sometimes claimed to be extraterrestrial beings.