There are emulators that use as input illegal copies of console
game ROMs (still under copyright). I guess that might raise some
eyebrows, and be considered *bad* by some/most people here.
Otherwise, on the *good* side, examples I can think of are:
DOXBOX - emulates DOS for games under WinXP
WINE - emulates Windows under Linux, therefore, can run Windows
freeware (I've used it -- a ways to go, but a good start)