While the DarkPlaces open-source game engine can already be used for some impressive games -- see Nexuiz and Xonotic as prime examples -- there's also the Twig Physics Library for the game engine to further enhance its physics.
The Twig Physics Library doesn't appear to have been updated in some time, but a Phoronix reader wrote in about this side-project, which was co-authored by "LordHavoc", the lead DarkPlaces developer that's now working for Valve Software on Linux.
Twig is a QuakeC-written physics library for DarkPlaces games and mods. Twig currently can handle simple rigid objects like crates/barrels and hanging/dangling objects while support for vehicles, gyro, and real rag doll physics as future features, assuming development picks back up.
More information on Twig can be found from its project page for the open-source library. Below is a short Twig demo video.
The Phoronix reader that sent in the notice about Twig had titled the email "Darkplaces with Half-Life 2 Style Physics!", but this is a rather over-ambitious statement considering the incomplete state of Twig and not supporting more advanced features yet like rag dolls, etc.
The reader went on to say, "There is no open source linux game with gameplay similar to that of Half-Life or Deus Ex where you could use the environment to your advantage by stacking stuff to get to higher places or killing your enemy by dropping a heavy object on his head. I believe the pieces to the puzzle are there to make a $60 game (like Bioshock or something) in a open source engine is there, somebody just needs to put the pieces together." The open-source pieces may be available to create a nice game, but one of the biggest issues is that most open-source game artwork/assets look awful. There's a real lack of quality artists within the open-source/community game projects with only a few titles actually looking good, such as Xonotic or Unvanquished.