hmm emus for mac or of mac...
if your searching emus for mac...
there are severals:
i think there is a mame version for mac
n64 emus: mupen64/sixtyforce (or something like that)
VBA is available for mac...
just look at google...
there are many emus for mac...
but if your searching for an emu of mac osx ...
there is no.. since there isnt any good ppc emu yet.
but if you have a ppc and use linux there is mol (mac on linux)