If you're a fan of Quake 3 then you may well find yourself quickly addicted to the multi-player tank shoot-em-up BZFlag.
The developers describe BZFlag as like a mix between 'Battlezone' and 'Tank Battle' which is a pretty fair assessment. Your aim, along with other players, is to drive around an arena and blast other enemy tanks. The 3D effects are much improved in this second release and do a good job of creating a fairly realistic multi-tiered battle arena.
These are no ordinary tanks that you're in charge of either. Tanks can jump, boost their weaponry and develop new battle techniques by capturing 'power-up' flags littered around the playing field. This flag retrieval aim is where the game gets its "BZFlag" name from. Tanks have to run over flags to gain powerups which enable them to drive through walls or use rocket launchers and lasers to blow away the most solid objects.
Generally, controlling the tanks is easy but when it comes to using advanced weapons, it's a bit more difficult because its hard to remember all the keys you need to use. Also, you can't play BZFlag on your own - its multiplayer only so you need an internet connection.
Give BZFlags a chance and you will see how addictive it is.