Move or Die might be my favourite party game, it's absolutely nuts, it can be played with by yourself and with others locally and online. It just had a big content update too.
You might remember I wrote about the gory FPS 'Apocryph' and that the developer was planning a Linux version, well, they've put out a test build for Linux already.
If you still haven't taken a look at HITMAN, now is certainly the time to do so. You can grab it from now until January 5th and get a permanent copy of the first episode set in Paris.
The FPS game Day of Infamy has a big 'Flak Tower' update out, which includes a new map and environment art. They also upgraded to a 64bit engine, but it seems the Linux version didn't get that.
GZDoom, the open source id Tech 1 engine that powers mods such as Project Brutality and standalone TCs like WolfenDoom: Blade of Agony or Castlevania: Simon's Destiny, has a new version out with improved support for Mesa drivers on Linux.