There are quite a few little different tetris games that'd you can play. Lots of puzzle games and little stradigy games.
In the 'games and amusement' catagory of packages for Debian Sid has around 808 packages. A lot of those would be seperate 'game' and 'data' packages. So there are probably about 300-400 different little games there.
A few are bigger, but most are just little things. Some bigger games would be like Flight Gear Flight Simulator, which is actually used in professional simulators, torcs, vdrift, battle for wensoth, tremulous, nexuiz, lincity-ng etc etc.
Then there are a few online MMORPG, mostly in development.
Like this:
http://www.planeshift.it/about.html
Now all I talked about was free software games for the most part.
Aside from that there are a handfull of native commercial games as well as lots of emulators. MAME arcade emulators. Dosbox for dos games. Emulators for sega genesis, snes, nes, atari, gameboy etc etc.
Some websites:
http://www.happypenguin.org/
http://www.linuxgames.com/
http://www.tuxgames.com/
http://www.linux-gamers.net/
Then besides games and such you can do things like setup streaming media servers, or setup a blog or CMS or whatever sort of website. Usually it's not very difficult to get things like Drupal going.