Humanshifting

Shape shifters are very common in fiction due to their flexibility. They can turn into a wolf, a giant, a tree, a whale, or sometimes only transform into another person.

How much a humanshifter can change varies. It could be just the face or it could be the whole body. The latter can lead to Gender Bender tropes. While Humanshifters don't have the versatility of being able to shift into another animal, they have their advantages. They are usually a Master of Disguise, able to replace their subject, and wreak havoc and many ways. Done rightly, they could bring down entire kingdoms without having to personally use a weapon.

See Doppelganger. Compare Shape Shifter and Master of Disguise. Contrast Wig, Dress, Accent. For nonhumans that become humans, then that's Humanity Ensues.

Examples of Humanshifting include:

Anime and Manga

In One Piece, this is Mr. 2 Bon Clay's power from the Clone-Clone Fruit. By touching the face of a person, he is able to impersonate the face, voice, and body, as he proved with Nami, of that person. There's a reason why he's ranked Number 2. It is also a hindrance, since to use his Martial Ballet Arts he has to use his real body, or something as strong.
Liar's Mask, Due's Inherent Skill in Nanoha Striker S, allows her to assume the appearance of any humanoid (though apparently only female of approximately the same height and build as herself).
Mirror Man, an agent of the British Library in ROD the TV Series.
An assassin in Vision of Escaflowne steals the appearance of several people (as part of the Kill and Replace routine).
Origami Cyclone's power in Tiger and Bunny.
Minor character Suetsumu in Zettai Karen Children, using telepathic hypnosis. An unusually benevolent example, since she uses it primarily to comfort terminally ill patients in her role as a nurse, and she doesn't even lie about who she really is while using it.

Comic Books

This is sometimes the Chameleon's power in Spider-Man, who is a Super Villain who has impersonated Spider-Man and some of his Rogues Gallery many times. (Depending on the Writer, this may be a shapeshifting superpower, or he may just be a Master of Disguise).
Depending on the Writer, Mystique from the X-Men comics is usually limited to human shapes (some adaptations have stretched this, though, such as the X-Men Evolution cartoon that had her turning into animals like wolves).
This is the power of the villain Everyman in The DCU.

Film

In Inception, this is Eames' job as a Forger, but he can only do it in dreams.

Live Action TV

The First Evil in Buffy the Vampire Slayer can only transform into real, dead (or ex-dead, or undead), people. You do see occasional glimpse of a demonic face that presumably represents its "true form," however.
X Files:
- Episode "Small Potatoes", Eddie Van Blundht has an extra layer of muscle that he can control to perfectly impersonate other humans. Moreover, although the episode doesn't specifically mention the limitation, we only see him in the forms of other male humans.
- The Alien Bounty Hunter can assume the appearance of any human he has met, e.g. Fox Mulder's.
- In the episode "Gender Bender", the villain shapeshifts from male to female and back, though s/he only seems to have a single appearance for each gender.
Victoria in No Ordinary Family had the ability to shapeshift into anyone else, even including their personality and superpowers. After she was killed, Stephanie was able to exploit this by temporarily impersonating her.
The Twilight Zone episode "The Four of Us Are Dying" features a con man with the ability to shape shift into anyone he chooses.

Literature

This is an ability that can be possessed by watercrafters in the Codex Alera. Even if you have only a limited watercrafting talent, you can change your appearance, though it takes longer to complete the process. Taking a form of a different size or shape from your own can be uncomfortable at best, though.
This is Tessa Gray's unique power in The Infernal Devices. Her whole schtick is that she can change her appearance into any humanoid, dead or alive, as long as she has touched an object that belonged to them. She's turned into both males and females.
Harry Potter:
- Metamorphmagi like Nymphadora Tonks (or her son Teddy Lupin) have the ability to alter her appearance at will, but must presumably retain a human shape. This is in contrast to Animagi, who can perform Animorphism into one specific animal.
- There is also Polyjuice Potion, which can turn you into a specific person for an hour, after you add a bit of said person into it (such as a hair, nail, etc.) It messes up if you add a non-human component, even if that person is a Half-Human Hybrid.

Tabletop Games

The Changeling race in the Eberron setting of Dungeons and Dragons has this as its signature power, being able to take on the appearance (and only the appearance, not the other innate powers of) other humanoids. By extension, the Doppelganger race has this same power too.
The alter self spell in Dungeons and Dragons lets you transform into another creature of your type. Since most spellcasters are humanoids, this means humans, elves, dwarves, etc. (Although a dragon casting alter self could only turn into a different dragon.)

Web Comics

Elliot in El Goonish Shive has this ability, although thanks to a botched magical awakening, most of his available forms are female. (Properly awakened magic in this setting tends towards Personality Powers, so it's been theorized that he won't awaken properly until he finds a female form that in some way represents his true self.) He also has access to a cat-person form, but the transformation is significantly more painful.
Succubi in Eerie Cuties 'verse can change appearance within humanoid variety, though the more changes (other than morphing away wings and horns), the harder it is to assume and maintain a given form. The latter became a plot point in Dangerously Chloe, when trying to impersonate a boy was quite an exercise for Chloe - mainly due to the breast size of her true form - and the effect of losing control was... rather amusing.

Web Original

Not uncommon among low-level Shifters in the Whateley Universe. Bogus, a student at Super-Hero School Whateley Academy can only turn into other people, and can't change his weight either.

Western Animation

A villain from Kim Possible named Camille Leon can shapeshift into people after an experimental cosmetic surgery.
Zee from The Zeta Project. He has been known to appear as a (humanoid) statue and a broken version of his robot form, otherwise it's strictly people.
False Face from Batman Beyond had the ability to change his facial features and hair to impersonate others, but was never shown to even appear as another gender or body type.