http://www.sco.com (which now in 2014 goes to http://www.xinuos.com/)
Embroiled right now in a lawsuit IBM (ScoVsIbm) that has upset the Linux community (ScoVsLinux).
Two major versions: OSR5 (a SysV, Xenix, BSD mosh) and Unixware, a SysV Unix.
Links:
http://aplawrence.com (TonyLawrence) FAQ, help files, reviews etc. AD
It is important to mention that SCO Unix is distributed by Caldera Systems and is current undergoing DoS attacks due to the above mentioned suite. Support can still be reached at http://support.sco.com Their main site has been moved to http://www.thescogroup.com as an alternative.
(See also VeSystemsInc)
Unbelievably, here it is late 2009 and this stupid lawsuit still goes on!