NewOS

NewOS is a free, full-featured operating system with full source code available, along with periodic binary snapshots. It currently runs on:

• Intel- and AMD- based single and multiprocessor desktops
• Sega Dreamcast

It is loosely based on concepts gleaned from BeOS, Solaris, FreeBSD, NT, and other reasonably modern OSes. The design goals are to be as modern and complete as possible, with an emphasis on platform portability.

http://newos.sourceforge.net/