Teeworlds: free online multiplayer game
Battle with up to 16 players in a variety of game modes, including Team Deathmatch and Capture The Flag.
You can even design your own maps!
I have been playing this now all night, all that i can say is WHAT A GAME.
This has to be one of the best,fun,retro style game i have played in some time.
Give it a go and leave a comment on what you think