ReactOS is an open-source operating system which aims to be 100% compatible with Windows XP.
Though still under alpha stage of its development, it already runs some popular windows softwares like Firefox, uTorrent and even games like Quake 3 Arena.

Though still under alpha stage of its development, it already runs some popular windows softwares like Firefox, uTorrent and even games like Quake 3 Arena.
ReactOS running Mozilla Firefox
Quake 3 Arena on ReactOS
ReactOS also is compatible with wine and is closely associated with the project.
Since still in alpha, portions of the OS are incomplete and buggy and hence the OS is not fit for everyday use. A beta version is planned for 2008 and it is hoped it would be more stable.
I would be very interesting to see the future of this project, considering that Vista is out.