ReactOS [L1 ] is a GPL 2 license operating system that aims to replace Microsoft Windows XP. Internally, it makes use of Wine libraries. Externally, it has a general look and UI similar to Windows XP in "classic" mode.
peterc 2008-06-10: While general system stability is a serious issue in version 0.3.4, my starpack applications ran on it without any serious problems, apart from some alpha transparency issues with some PNG icons in Tile buttons.
It's promising in that by the time it is a stable platform in a couple of years (say, in 2010), it will be in competition with Windows 7 without MinWin to stem the Vista bloat. ReactOS may be an easier sell to IT departments at this time than official Microsoft Windows.
peterc 2008-06-13: getWindowsAPIVendor is a small but handy proc for detecting which win32 code base your app is really running on: Wine, ReactOS or Microsoft Windows. This can be used for tuning your app or informing the user of support/non-support.
Looking at the web site, the effort is currently working on version 0.3.13. It has gone through 0.0.8 releases in 3 years.
AMG: I have to give them credit for posting Cave Story screen shots. ;^) Though, the font rendering is quite funky: http://www.reactos.org/media/screenshots//2009/ros_039_cave_story.png . (Dear Wikignomes: please fix the preceding link to show up as  or similar, without being displayed inline.)