HaxBall is a free to play flash game available at http://www.haxball.com. The game has a community of players organised into local, national and international leagues.
From the website:
What is HaxBall? HaxBall is a realtime multiplayer game that plays like a mix between football and air-hockey, and it's a real blast. Best matches are 2-vs-2 or 3-vs-3, but there's no limit to how many can join.