http://www.thedarkmod.com/The Dark Mod is single-player a total conversion mod for Doom 3 inspired by the Looking Glass Studios stealth games Thief: The Dark Project and Thief 2: The Metal Age. It builds upon the gameplay of those games and even improves upon it in many ways. If you liked the first two Thief games you owe it to yourself to play this, as it's probably the closest thing to a modern equivalent to the earlier Thief series there is.
Recent additions with the 1.07 update include:
-Vine Arrows
-Improved AI behavior, especially regarding doors and rope arrows
-New combat sounds and animations
-Fixed issues with mission installation and saved games for Steam users
If you've never played a Thief game, then this is perhaps best described as a stealth game set in a somewhat medieval gothic steampunk fantasy setting. In most missions you play as a thief, and you steal things (duh).
There are a several missions already available for The Dark Mod that can be downloaded from the site or through the in-game downloader.
A few of my favorites that I highly recommend:
-The Knighton Manor
-The Caduceus of St. Alban
-The Beleaguered Fence
-Mandrasola
-A Score to Settle
-The Heart of Lone Salvation
A gameplay video of the mission "The Caduceus of St. Alban":
http://www.youtube.com/watch?v=d-fon8KWqn8&hd=1
