The Dark Mod is an awesome tribute to the Thief series, but often goes unmentioned in threads like this for some reason. Arguably, it's even better than Thief at this point, and has new content added on a regular basis by it's community.

It used to be (I believe) a Quake mod back in the old times, hence the name, but it's long since been transitioned to a standalone game with a different engine.