VAX emulation is fast and complete. Requires hobbyist license to get OpenVMS running, but worth the effort. Also includes 8-bit CP/M hardware emulator.
Any fans of Big Iron™ should get a kick out of SIMH. I've used it to relive my student years on PDP-8 and PDP-11 machines running a variety of systems (including my all-time favourite: RSTS/E).