Some things that aren't really gaming, or are marginal, but deserve mentioning.
There are programs that emulate your favorite old arcade machines, and are available for Linux. There are some legality issues. The emulator is perfectly legal, but the ROMS of the games themselves are only legal if you own the games hardware. If you do not own the games hardware, you are breaking copyright.
The Multiple Arcade Machine Emulator for X-Windows. This plays all of your old favorite arcade hits. Pac-Man, Missile-Command, Bubble-Bobble, Mortal Combat, and literally thousands of others. http://x.mame.net/
Emulates an Atari 2600. Remember Combat? Pitfall? River Raid? These can come back to life for you. http://stella.sourceforge.net/
Having one of these will allow you to hook your Gaming Console (PS2, GameCube, XBox) to your Linux box, using the monitor and (hopefully) expensive speakers to show those games. For a list of supported cards, see the Linux Hardware Compatibility HOWTO. http://www.tldp.org/HOWTO/Hardware-HOWTO/index.html