It's a multiplayer tank-driving flag-collecting team based shoot-em-up. It's pretty popular on the net, but as its a free open-source game its probably not as well known as some of the commercial stuff.
I'd love to see a local server for this if there's any demand. The server runs on linux, unix etc, I'm sure it's not too heavy duty.
We could do with some UK servers, the yanks have a bit of a monopoly.