The Dark Mod is a modification that turns PC shooter Doom 3 into something a little more akin to what fans of the Thief series are used to. More men in armour, less monsters in closets.
While it's been out for a while, it recently received a big update, and in promising news may get another one soon; right now you need a copy of Doom 3 to play it, but with Doom 3's source code about to hit the public domain, its makers are looking to take that code and turn The Dark Mod into a standalone title.
The Dark Mod [ModDB, via Rock, Paper, Shotgun]