THE DARK MOD is a free, first-person stealth game inspired by the Thief series by Looking Glass Studios. It recreates the essence of the original Thief games on a more modern engine. This project is both a game and a mission-creation platform, The Dark Mod allows players and mappers alike to enjoy missions in a gothic steampunk universe.
The Dark Mod is built off the the Doom3 game engine. Doom3 is a first-person shooter from "id" software, whom also made the FPS Quake, which allot of open-source games that run on Linux are based off of. After a certain period of time the id software company re-releases their game engines as GPL open-source which allows indie developers to start making games with the engine they just can't use the same artwork.
The Dark Mod was officially released on October 17th, 2009. As of October, 2013, TDM 2.0 is now completely standalone, and no longer requires Doom3 which is the game engine used to develop The Dark Mod. The newest release on January 29, 2014, TDM 2.01 has improved the graphics and physics quality.
Here is a trailer for the 2.00 release which shows some pretty cool game mechanics.
Installing The Dark Mod
Install TDM on Linux
Install TDM on Windows
The Dark Mod download page (in case the links posted above are broken).
Check out their wiki for more information