Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application that just happens to contain the most powerful engine this side of a million dollars.

Unity 2.1
Unity has evolved to be the perfect MMO and virtual world client, both in the browser and stand-alone. Stream in limitless landscapes as your users travel your universe, customize characters, and load infinite locations and virtual items on demand. When Funcom, makers of Anarchy Online and Age of Conan, decide to use Unity for their upcoming browser based MMO project, you know it has to be good.

Unity can be purchased by indie developers only for 149 euro!!

The only downside (as far as i know) it’s that you can publish on many platform, pc, wii, iphone BUT you can develop only on the Macintosh platform. BUT as you can see from a guy comment down here, the PC version will (should) be available very soon.

LINK