ReactOS

ReactOS, originally beginning life as FreeWin95, is an open-source implementation of the Windows NT architecture, incorporating several other libraries such as WINE, and many extensions of functionality such as Ext2 support. ReactOS also boasts a port specific to running on the Xbox, although this requires a modified BIOS.

Because the operating system's nature is one of constant development, it can serve as an ideal testing platform for video game compatibility.