NetBSD

http://www.netbsd.org/images/NetBSD-smaller.jpg

NetBSD is a free, secure, and highly portable UNIX-like operating system available for many platforms, from 64-bit AlphaServers and desktop systems to handheld and embedded devices. Its clean design and advanced features make it excellent in both production and research environments, and it is user-supported with complete source. Many applications are easily available.

http://www.netbsd.org/

http://www.netbsd.org/Documentation/

Also see: BsD, FreeBSD , OpenBSD