Team fortress 2

Its been freeware since june, so go and grab your copy from steam!

This game has really hooked me in, I haven't had so much fun fragging since the orginal tf, counter strike and maybe enemy territory The game so enjoyable because you can just jump in easily. No need to plan or anything, just go with the flow. Like all the best games, its easy to learn, but hard to master The game balance is very good and there are lots of servers.