A few noteable ones to add on to the ones BC mentioned:
There is Solaris which runs Sun's SPARC workstations and servers, OS/400 which run's IBM's AS400 servers, and Amiga, though I'm not sure what that last one runs
, but there are a ton out there, more Unix / Linux / BSD variations than you could possible try.
There was once an article in Maximum PC magazine where someone sucessfully installed and was able to boot to 70 some odd different OS's on his PC at once through boot loaders and about 8 drives.