SIMH is a highly portable, multi-system simulator.

SIMH contains over 40 individual old school computer emulators/simulators.

A short list of some of the emulated/simulated systems:

Data General Nova, Eclipse
Digital Equipment Corporation PDP-1, PDP-4, PDP-7, PDP-8, PDP-9, PDP-10, PDP-11, PDP-15, VAX
GRI Corporation GRI-909, GRI-99
IBM 1401, 1620, 1130, 7090/7094, System 3
Interdata (Perkin-Elmer) 16b and 32b systems
Hewlett-Packard 2114, 2115, 2116, 2100, 21MX, 1000
Honeywell H316/H516
MITS Altair 8800, with both 8080 and Z80
Royal-Mcbee LGP-30, LGP-21
Scientific Data Systems SDS 940
SWTP 6800