The Dark Mod 2.0 is free and open-source and with version 2.0 has been made completely standalone to no longer depend upon Doom 3 assets -- in other words, you don't need to own a copy of Doom 3. Back in 2012 we wrote about The Dark Mod on Linux.
Besides being made standalone, The Dark Mod 2.0 has AI improvements, new gameplay, audio improvements, graphical improvements, and updates to missions.
Sadly with all the interest in the new game release, TheDarkMod.com is down but a Phoronix reader pointed out that the 2.0 release information is currently mirrored on Reddit.
The Dark Mod 2.0 is now one of the few successful open-source game projects on the Doom 3 engine (id Tech 4). Making me particularly excited today and wanting to try it out is to now have a standalone completely free game on the id Tech 4 engine to use for benchmarking. Doom 3, Quake 4, and Prey can all be found with the Phoronix Test Suite and OpenBenchmarking.org, but require a game license to work. Now The Dark Mod 2.0 will hopefully work for some new open-source GPU/driver benchmarking needs on Linux.