The Dark Mod has long been one of the most famous PC mods around. Using Doom 3 as a base, the idea was to transform id's shooter into an old-fashioned game inspired by the old-fashioned Thief games. It was great. Now it's even better, because it doesn't need Doom 3 anymore.
Version 2.0 of TDM is now out, bringing all kinds of nice changes and tweaks to the project. Best of all, though, is that it's now a full standalone release. Oh, and if you're new to it, it's free.
The base package is around 2gb, with missions to be added on top of that. You can get it here.
The Dark Mod [Site]