With an interface very similar to that of iTunes, OpenEmu will allow you to remember times gone by no matter what consoles you own, since you can play virtually all of them. Now that all changes with OpenEmu, the definitive retro emulator now available for Macs. Also, if you want to play different consoles, you must have different emulators. But the vast majority have pirate origins and it shows in their interface and their complicated game systems.
Obviously, there are already many emulators available on the net.