The Cradle from Thief 3, (one of the most brix shitting experiences of all times), I should point out that there are a lot of terrific fan missions for thief 2, some of which are better than the original game, and many of which are unbeleivably brix shitting.
Both spots to grab plenty of the missions. You need some extra stuff, Darkloader (can get everything you need at the Keep of Metal and Gold)
Brawl in the Tombs
Island of Iron parts 1 & 2
What Lies Below
The Inverted Manse
All great missions. Deceptive Perception will traumatize you, and the Inverted Manse is one of the most consistantly creepy levels I've played for any game (fan made or not). It compares with the cradle from thief 3, but does come up short in terms of sheer gut wrenching terror.
Deserving special mention though, is Calendra's Legacy, a 350 meg 3-level fan made mission pack that just blows the shit out of major releases in terms of sheer quality entertainment.
The second mission is all about zombies too.
Worth playing if you like the thief series, or don't mind slow-paced cereberal gameplay from time to time.