Linux IBM
Linux at ibm. there are now more than 15,000 ibm linux customer engagements worldwide, allowing customers to reduce their computing costs with solutions ranging from web serving to some of the largest supercomputers.
http://www.ibm.com/linux
FreeBSD
The freebsd project
http://www.freebsd.org/
AIX 5L (IBM)
Aix is ibm's industry-leading unix operating system that meets the demands of applications that businesses rely upon in today's marketplace.
http://www.ibm.com/aix
Linux on z/VM (IBM)
Ibm z/vm and linux on system z resources
http://www.vm.ibm.com/linux/
OpenBSD
The main openbsd page
http://www.openbsd.org/
SuSe Linux
SuSe Linux
http://www.suse.com/
Sun Solaris
The solaris 8 operating system data sheet
http://www.sun.com/software/solaris/ds/ds-sol8oe/
RedHat Linux
RedHat Linux
http://www.redhat.com/
UX (HP)
Hp-ux 11i home for hp's unix operating system, delivering leadership unix value through availability, security, and virtualization.
http://www.hp.com/products1/unix/operating/index.html