Toribash is a martial arts-based ragdoll physics game developed originally by Hampus Soderstrum in Sweden (Who has now left the development, I think, but still helps out) that is for Mac, Windows, and Linux. Huge community with online, and a fairly addicting game. HOWEVER, it's extremely hard to get the hang of.
It's also moddable, with a crapload of mods preinstalled. If you win a match in multiplayer, you get Toricredits(TC) to spend on things like colors for your Tori (What they call the avatar thingies). You can also buy TC, but I'd only reccommend that if you have the spare money.
It used to be pay-to-play, but it's now free. Not sure how long that'll last, though. Might be forever, might not.
Just being in online matches gets you QI, win or not, and if you get enough QI you advance in belt(Sort of like leveling up, except there are no stats, so every tori is the same. Making skill being the true deciding factor. Nice)
Site: http://toribash.com/
Video(Replays of other players online): http://www.youtube.com/watch?v=2FMtQuFzjAc
