Want to play Thief on Doom 3's engine?

[Hannibal]

Darkmod is totally awesomeness. Very easy to install but you need the original Doom 3 not the BFG version. Has its own download client which makes downloading user made missions easier. Great way to extend your Doom 3 purchase.

http://www.thedarkmod.com/main/

It also has its own editor for the creative types. Total install for the Darkmod is 2 gigs.