Tons! For starters, here is an (out of date) list and here is an article about someone's top 25 Linux games for 2008.

Websites dedicated to Linux gaming:
http://www.linuxgames.com
http://www.happypenguin.org
http://www.tuxgames.com

Besides native games & ports there are all kind of emulators. There is even an implementation of the Windows API (WINE) which allows one to play Windows games with variated success.

However, one might want to run a specific game on Linux.

The focus on the NIT is to get all kind of classic games (and emulators) running. This opens up possibility to get a huge arsenal of games working on the NIT however mostly 'old' games or clones of old games. Useful for several reasons e.g. nostalgia but also if the game formula is well or aspects of the game are good (e.g. storyline in Beneath A Steel Sky) it is potentially a lot of fun.