I know some but tell me if I missed any.

Cobalt
Unix
Linux
Solaris
Sun
IRIX
WinNT


hmm...

UNIX is kinda of broad
and sun and solaris are the same thing
BSD
FreeBSD
BSDI
OpenBSD
Redhat
Turbolinux
Debian
Mandrake
Corel

You want more??

And let's not forget the 2 or 3 servers running on an Apple OS. ;-D

Duster....check your mail.... ;)