rezzy...you are FULL OF YOURSELF... ;) I like you anyway.
Whoops! That double post was inadvertant...I do not want to give rezzy a big head.
actually, no, i'm not...
Hercules is a S/390 emulator.
Sheepshaver emulates the PowerPC G4 processor (minus MMU), and thus enables one to run Mac OS 7.5.x through 9.0.4 so long as one possesses a legal Mac ROM image.
Simh emulates a bunch of historical systems such as the PDP-11.
Qemu by default is a complete PC emulator (simulating a PC with a 800MHz Pentium II) and can run anything between DOS 3.3 and Windows XP, plus various forms of Linux, BSD, and even OS/2. (XP and later would probably use VMware's free VMware Server). Though I can also emulate Alpha, SPARC, Linux/PowerPC & RISC, ARM, etc.
and finally, I'm running openSuSE 10.2 as my flagship OS.