Microsoft's UNIX version. Earliest versions were based on System 7 UNIX and released in 1980. Later, Microsoft sold the system to SCO, where it became first SCO Xenix, then SCO UNIX, and in the 1990s, SCO OpenServer (which, according to those who have used it, stays firmly in the 1990s and won't budge without a massive investment.)
Even though I haven't used Xenix (many say this is just a positive thing), I might tell here about one big legend I've heard:
Xenix's fsck command, for some obscure reason, asks for scratch file name. Many sysadmins, upon confronting this problem, just write the filename that's printed just before this question - something like /dev/hdroot0...
"Must be a fast machine as fsck ran quick..."
Microsoft has continued this tradition of making intuitive user interfaces ever since... =)