Wanna try some new games? Here are our top recommendations:

Game	Genre	Remarks
bzflag	3D tank Shooter	Drive around in a tank and shoot at other tanks. Get a flag that makes you invisible, fast, agile or something else.
freeciv	strategy	Be the leader of a civilization and conquer the world. Decide if you want to go to war, develop your science or build world wonders and so on.
AngryBirds for chrome		Fire birds to destroy buildings :)
ksokoban	tactics	Move boxes, but do not let the boxes get surround you.
pingus	tactics	Lead your pingus from one end of the dungeon to the other. Have them build tunnels, holes and so on.
frozen-bubble	tactics	Shoot the bubbles together by color
Battle for Wesnoth	strategy	Turn-based strategy game with multiplayer over Internet/LAN and hotseat mode (multiple players playing on one computer).
Neverball	action	Tilt the floor to roll a ball through an obstacle course.
OpenTTD	simulation	Open source clone of Transport Tycoon Deluxe with many extension. If you liked Transport Tycoon, you're going to love this.
xboard	strategy	Chess for Linux

[edit] Sources

There is a great german page on linux games http://www.holarse-linuxgaming.de/ with lists and screenshots

[edit] Jochens favorites

0ad, online realtime strategy http://wildfiregames.com/0ad/
The quake3 based egoshooters Open Arena http://openarena.ws/, Alien Arena http://red.planetarena.org/ , Urban Terror http://www.urbanterror.info/ and World of Padman http://worldofpadman.com/ , the latter a great multiplayer game made with much love and great design
and of course flight gear the flight simulation http://www.flightgear.org/
as well as little games like e.g. foobilliard http://foobillard.sourceforge.net/ or PokerTH http://www.pokerth.net/