For any fans of the old thief games, or fans of stealth games in general, check out The Dark Mod -

http://www.thedarkmod.com/main/

Its a completely free, standalone first person stealth-em up, as a long time fan of stealth games, I highly reccomend it. Some missions are incredible immersive.

Some gameplay - https://www.youtube.com/watch?v=j_14y8ztA8k