ReactOSWiki

Description
"ReactOS® is an advanced /Open Source operating system providing a ground-up implementation of a Microsoft Windows® XP compatible operating system. ReactOS aims to achieve complete binary compatibility with both applications and device drivers meant for NT and XP operating systems, by using a similar architecture and providing a complete and equivalent public interface."

As of early 2007, ReactOS is still in alpha stage, so it is not recommended for everyday use. Developers use the wiki to coordinate work on making it usable.