The Chitauri, from Marvel Comics’ Ultimate line, although they’re basically alternate universe versions of the Skrulls mentioned upthread.
Reptilians, from modern UFO folklore (David Icke, et al).
Serpent Men, from Robert E. Howard’s King Kull and Conan stories, although they’re a prehuman terrestrial species, not extraterrestrial. Also, although they can’t quite perfectly imitate humans - they can’t say the phrase “Ka Nama Kaa Lajerama”.
He’s a Green Martian, already mentioned upthread by Kamino Neko.
Two more from Marvel Comics:
Poppupians (exemplified by Impossible Man) originally could perfectly imitate virtually anyone or anything. In later appearances they couldn’t change color (so they could perfectly imitate a human except they would be green).
The Technarchy, “techno-organic” beings with…inconsistent shapeshifting abilities, exemplified by Warlock of the New Mutants. In at least some appearances, they could flawlessly imitate humans.
Skrulls were the first thing that occurred to me for some reason. Seems like about half of all alien species can shape shift to human form, and half of the rest already have human form.
[Church lady]How convenient![/Church lady]
Since we’re of course mentioning fictional aliens, maybe someone should bring up the space zombies from Pluto. “It’s already mutated into human form! Shoot it!”
Scarlett Johansson plays an alien in human form in Under the Skin (2014). Other than seeing Scarlett Johansson nude, there’s not much reason to watch it. 
The unnamed aliens in Matt Haig’s novel The Humans.
But was Johansson’s character a shapeshifter, or did her natural form just happen to resemble a human?
There’s a part at the end where she sheds her human skin. It’s never really clear what she is, but she’s been formed to resemble a human.
Cocoon
Animorphs - not limited to humans
My Favorite Martian - movie version
Contact - “a form you’re comfortable with”
Dreamcatcher - not the villainous ones that need to possess humans, but the good one that opposes them
V
Can’t tell you how many times I’ve said this about old girlfriends. :smack:
X-Files?
Did I miss it? Or was it mentioned? Surprised it wasn’t suggested early on, or does X-Files not count?
Also, the guy from UHF. ������
Wasn’t there a shapeshifting assassin in one of the Star Wars prequels? Attack of the Clones, I think?
And the female lead in Cowboys and Aliens usually looked human, but IIRC we get a glimpse of her true form (and she admits that her true form doesn’t actually look human).
If we’re talking superhero media (as the Skrulls being the only example suggests) it’s the other way around. More than half look passably human (non-blue Kree, Shi’ar, Kryptonians, Thanagarians, etc) and probably half of the ones that don’t can shape shift (Skrulls, Martians, Durlans, etc) , or easily disguise themselves as human with a bit of makeup (blue Kree, Coluans, etc).
Many of the monsters & spirits in the webcomic White Noise can shape-shift into a perfectly passable human form. Some can shape-shift into other forms as well.
The ones in Cocoon and V (like the Slitheen) wear special suits to look Time Lord, so not shape-shifters.
You say that as if I need another reason.
From DOCTOR WHO, Carrionites “resembled a giant skeletal raven or crow”, but one who messed with William Shakespeare “could assume the form of a beautiful young woman” and worked as part of a trio — and you get the idea, right? When wearing a hooded robe, can morph to a form that could pass for ‘old hunched crone with a beaklike nose’; uses alien tech that’s built around biological samples and voice commands, which looks a lot like ‘getting a lock of hair from someone before casting a magic spell’; and their other accoutrements run the gamut from ‘oh, the crystal happens to be shaped like a ball’ all the way over to ‘okay, that right there is clearly a cauldron’.
Lyekka, the carnivorous plant in Lexx