First beta has been released a couple of weeks ago and I had a good time playing the first fan missions.
If you are a fan of the Thief series or other stealth games, you should give it a try: http://www.mindplaces.com/darkmod/

Q: What is The Dark Mod?

A: The Dark Mod is a total conversion mod for Doom 3, which recreates the moody stealth experience reminiscent of the 'Thief' games from Looking Glass Studios.

Q: So is it a game?

A: Technically, no. It's primarily a toolset, allowing fans to create their own missions in a medieval steampunk environment. When you download the mod, the only thing you can play immediately is the Training Mission. You have to download other missions separately (see our download page for more details).

Several missions have already been released for download, and more will be completed in the weeks following the release.

Q: What do I need in order to play?

A: Other than TDM itself, you must have a copy of Doom 3. TDM works with the steam version of Doom 3 if you can't find one locally. You must also have the most recent D3 patch.