Been watching this one progress via the TIGSource forums and it's now definitely at the point where it's worth mentioning. Realm of the Mad God, created for the TIG Assemblee competition, is 'a massively co-op fantasy adventure' featuring a remarkably large world to roam and explore (inhabited by thousands of monsters), real-time battling, a full leveling up system, lots of different equipment and magic spells to experiment with and 5 different classes to choose from.

All this content, and it's still being built. Starting a game is as simple and entering your name and pressing go, at which point you'll be dumped into the world alongside every other connected player. You can choose to team up (text chat is available) or go it alone.

A major thing to note - death is permanent. Lose all your health and you'll have to start all over again, whether you were level 1 or level 100. In fact, in this respect it reminds me a lot of Farb's Captain Forever - you may be able to get to a respectable level, but make little slip and it's all gone.

Go give it a try, and make sure to follow the progress over on the TIG forums.