Although I'm not a "gamer", I enjoy playing computer games from time to time. First-person shooters are especially well-suited for relaxing after a long and exhausting day ;-)
IMHO a very good choice, if you're looking for a fun Free Software first-person shooter running on Linux, is Nexuiz.
Nexuiz version 1.5 has recently been released, which features new maps, new characters, better AI and lots more. Check it out!