Thief: The Dark Project turned 20 years old this week—if you missed Richard's piece on how it defined the stealth game, then you should really give it a read. To celebrate the occasion, the community over at Through The Looking Glass released 24 new fan-made missions as part of a design contest, and you have until March 1 of next year to vote for your favourite.
The creators all had a year to build their maps using the game's level editor, and could only use stock assets. If you want to get started, it's best to start on this forum post, which has instructions on how to download all the levels and score them for the competition.
From that page, you'll also be able to click through to individual forum threads for each mission. If you don't know where to start, reading the comments on those posts might give you an idea of which ones you'll enjoy, as well as some pointers for if you get stuck.
To play the fan missions using either Thief: The Dark Project or Thief: Gold, which is the updated version for sale in digital stores, you'll need to TFix, an all-in-one patch. Instructions on how to do that are here. As mentioned on the Through The Looking Glass forum thread, you should disable all the optional features TFix offers, such as Thief 2 textures, and avoid using any other enhancement packs or HD texture packs.