This list might be up for discussion but I think the best emulators on the mac for the following consoles are the following (note many of these are ports from Windows apps so compatibility should be the same). I'll put a (b) for emulators which are by Richard Bannister http://bannister.org
MAME OS X - The newest MAME app for OS X. Its built from the MAME core and supports some nice OS X touches. Works well on PowerPC and Intel. http://mameosx.sourceforge.net/
SDLMame - no gui for this but its just like the linux SDLMame so support and advancement is good. http://rbelmont.mameworld.info/?page_id=163
MacMAME - http://macmame.org (outdated but still pretty popular since it has been around for ever).
Nes - Nestopia (b)
Genesis - Genesis Plus (b)
BSNES (b) (this app is very processor intense as it is cycle accurate, but if you have a G5 at least its worth it)
SNES 9X (for slower machines but still very popular since its built from the windows SNES 9x) http://www.geocities.co.jp/SiliconValley-PaloAlto/2560/snes9x.html
Atari 2600 - Stella
N64 - The N64 scene is not up to speed on the Mac the way it is on Windows, but still many of the popular games can be played at high resolutions with one of the following apps.
Sixty Force - http://www.sixtyforce.com/
Mupen - http://mupen64osx.lamer0.com/index.htm
There are emulators for just about everything else out there, these are just the ones which I pay attention to. You can find a much more complete list at http://www.emuscence.com
Here is is in all its direct link glory
MAME - Multiple Arcade Machine Emulator
The current version recognizes over six thousand ROM sets.
Civilization 1 and 2
ZSNES - Alexandre
Snes9x.com - FleetofFoot
also try http://www.eidolons-inn.net/tiki-index.php?page=Kega
Mac emulators -Velhetica wants to know
Haven't tried any but here are some lists
Varies from country to country
It is illegal to possess a ROM image file if you do not own the original ROM.
However, in the U.S. it has been illegal since 1983 for a user to create their own backups of video game ROMs. This was decided in the court case of Atari v. JS&A
Some ROMs are freely licensed.
Copyright holders have the right to profit or not profit on their creation.