Here is a really fun, challenging and very cool game: Tux Racer. Your aim is to guide the Linux mascot down snow-covered mountains and collect as many herrings on the way as you can. The game is available for Linux, Windows and Mac and is open-source.

The game is really challenging, the trick is to keep your speed down, otherwise it becomes impossible to grab all the fish!

Download: http://tuxracer.sourceforge.net/