- SuperTuxKart
I really like this game because it's basically just FOSS Mario Kart. It has a lot of gamemodes, an (admittedly simple) kart stat system, many tracks and a lot of mods.
- OpenTTD
In OpenTTD, you transport cargo and people to different places. You can transport them using trains, buses, aeroplanes (airplanes if your American), boats and trams if you add a NewGRF (a mod). It is very addictive and it is quite easy to get into (except the signals).
- Pingus
It's FOSS Lemmings with penguins.
- Extreme Tux Racer
You play as Tux (or someone else) and slide yourself through terrain while you collect fish. It is fun, however I've found that it can get a bit repetitive. You can design levels, however it's a bit hard to do.
- SuperTux
It's 2D Mario with a penguin. BEST GAME EVER.
http://crawl.akrasiac.org:8080/#lobby -- leading edge of rogue development