FreeBSD
The freebsd project
http://www.freebsd.org/
Linux
A means to world liberation! our goal is to provide all the information necessary to make your use of linux a success.
http://www.linux.com/
Microsoft(windows)
Microsoft(windows)
http://www.microsoft.com/windows/
OS/2
OS/2
http://www.software.ibm.com/os/warp/