Through the Looking Glass Forums
This is the largest place for all things Looking Glass Studios made and their related games. The Editors' Guild comes with a vast archive of DromEd wisdom (in case you ever want to touch the beast that is the Thief editor).
To play Thief on modern machines, you'll likely need to install Timeslip's ddfix patch which fixes several technical issues.
The Weary Taffer
Home of Yandros, author of popular fan missions like The Burrick's Head Inn or What Lies Beneath. He's also hosting his own forum which fan mission authors can use for private beta testings.
The Nameless Tower
Nameless Voice's website features a lot of custom objects for your Thief levels. He's also created an outstanding custom script module that allows you to do things in DromEd that the original designers didn't even dream of.
Shadowspawn's Thief Pages
If you want to create your own objects and AI meshes for Thief, Shadowspawn's utilities are a must-have.
This site hosts nearly all fan missions in existence and also provides a platform for people to write reviews of Thief FMs.
Keep of Metal and Gold
Another nigh complete FM archive. Komag has organized a lot of FM contests which always render a bunch of interesting bite-sized missions.
The Dark Mod
This Doom 3 total conversion is still in development, but already now one can tell this is going to be great. It's essentially Thief in a newer engine with features that open a lot of possibilites to mission designers.
Adventure Game Studio
This is (in my opinion) the best choice for making adventure games in the classic low resolution of 320x200 pixels. If you don't like the Sierra style interface, you have to do some scripting or use one of the available modules made by other AGS community members.
Wintermute supports 2.5D adventures (realtime polygon characters on pre-rendered backgrounds), though you can still make them purely 2D as well. The engine's scripting language is a bit more flexible than that of AGS.