BZFlag is a free and open source multiplayer tank capture the flag game.

I’ve been playing it quite a bit over the last few days and it’s certainly quite different from other games I’ve played – you navigate by positioning the mouse cursor towards the top of the screen to accelerate, to the left to turn left, etc. You drive around a map destroying enemy tanks. You can also capture flags which give you special rewards for example making you smaller, bounce more, invisible except on radar, etc.

For some reason, tanks can also jump. 

BZFlag is available on Linux and Windows.

• The "Games" category of the blog has links to some other free and mostly open source games.