Team Fortress 2- the definative online shooter.
I bought Team Fortress 2 on a 33% off Steam weekend, hoping it would be a bit of fun for AU$20, but instead it has pulled me away from Call of Duty 4 and become my favourite online game. The 9 classes each play completely differently, meaning boredom is very hard to find.
Valve are releasing excellent content packs for each class, featuring a bunch of achievements to unlock 3 new weapons, so far Medic and Pyro have been released, with Heavy set to come next.
The game runs brilliantly on my 8800GT at 1920x1200 resolution, full settings, and the graphics look beautiful, the maps are well designed and all game modes are fun and satisfying.
There are a great range of low ping servers for Australians which is nice, and 24 players isn't too crowded on any maps.
If you like FPS, or even just like it a bit, or don't like it, buy Team Fortress 2, the content packs mean this game just gets better and better!