Exitus

Exitus is a third-person stealth and action game where you play an alien who, after being captured by humans and confined to a spaceship, must escape no matter what it takes.

This alien, unlike most aliens, has no advanced weapons or superhuman strength. Instead, it has the ability to mimic other living beings. To keep from getting noticed, you have to assume the form of the humans you see on the way.

For most of the game (which is not particularly long), you'll need to use stealth to pass watchtowers and sneak up on security guards, then disguise yourself as one of them.

Exitus is a fun stealth game with an approach that's radically different from most games of this type. It also has excellent graphics, considering it's a free game developed by students.