ReactOS

Preferred license: GNU General Public License (GPL)
The ReactOS® project is dedicated to making Free Software available to everyone by 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.
Actual source code produced by the student participants in Google Summer of Code™ for ReactOS can be found here.
 
Current Projects
by Michele Cicciotti, mentored by Alex Ionescu
by Pablo Borobia, mentored by Thomas Weidenmueller