Unity has announced that their Unity 2.5 IDE is now cross platform and now works Windows in addition to MacOS.
Unity rebuilt the entire Cocoa-based UI that they had previously with a Unity-powered UI. The entire UI is now built in C# using the Unity built-in APIs (all the controls, views, widgets).
This is a little bit like a compiler compiling itself. This time it is an IDE built using the IDE itself
Hot Off the Presses: Unity Goes to Windows