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 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.
You can develop for Windows, Mac, IPhone and Wii.

Unity 2.5 adds full support for Windows Vista and XP, with 100% feature parity and interoperability with Mac OS X. The Unity Editor has been rebuilt to look, feel, and function identically on both operating systems, each running the same underlying engine. The best part? Unity on either platform can build games for either platform — cross-platform in the truest sense.

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

LINK