Windows (NT4 the precursor to 2000 and XP) ran on Alpha, Mips, and Sparc, later versions run on Itanium too.
Macs have run on 68000, PowerPC and x86
Solaris (Sparc and x86)
AIX (Itanium, Power PC and others)
HP-UX (PA-Risc and Itanium)
Amiga OS (68000 and PowerPC)
And in the embedded market
I think the trickier question is to ask, which 'real' operating systems only supports one architecture?