1. 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.

2. 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).

3. Pingus

It's FOSS Lemmings with penguins.

4. 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.

5. SuperTux

It's 2D Mario with a penguin. BEST GAME EVER.