F2P MMOs are the only MMOs I always say, and my absolute favorite is the side scrolling arcade style beat'em up that is Dungeon Fighter Online. I was really pulling for it to win MMO Game of the Year last year but things just didn't work out that way. Anyhow, the game is coming to the Xbox 360 via Xbox Live!
That doesn't even begin to scratch the surface of why this is insanely awesome though. It is also set to be the first ever free-to-play title offered on the service. Similar to the PC version, you can download and play for free, although extra things like costumes, pets, etc. will need to be purchased. There are no solid release details but the game will be launched on a global scale and in eight languages. This is the first game I've seen since making the switch that I honestly think warrants purchasing an Xbox Live gold account.