Are you asking for mods or fan made missions? And have you played the game before or is this the first time? There aren't very many mods that enhance the existing game. There are a number of patches that are useful, including some mission fixes which you can find at http://www.thief-thecircle.com/.
The only mod I've ever made use of is the "Thief Enhancement Pack" which basically adds some better textures to the game.
I also can't help but recommend playing Thief 2x if you haven't. It's a production quality game.