If you are into games and haven't played or even heard of Thief, you've got to do yourself a favor and get a copy of both Thief 1 & 2. Seriously.
Due to their age the games can be a bit iffy to get to run properly nowadays (especially Vista renders some "funny" results), but the required effort is certainly worth it. And when you've finished the games, there are hundreds of fan missions (FMs) waiting to be explored.
In this section you can find all Thief 2 FMs I've created so far with descriptions and screenshots. If you are an FM author, there's also a section where you can download custom assets for your own missions.
Now, why would anyone want to make missions for such an old engine when there's sparkly technology like Unreal or Radiant (of Doom 3 fame) around?
For me Thief is a unique experience that hasn't quite been reached by any other game ever since. Thief comes with an intriguingly anachronistic setting and offers a distinct gameplay blend of stealth, freedom, and puzzle solving. Although puzzles certainly aren't Thief's main ingredient, they were one reason why I started making FMs. I like designing adventures (as you might have guessed already from my games section), and Thief 2 allowed me to integrate puzzles into the world and counterbalance them with the stealth elements.