All main Amiga Emulators are based on UAE, the Universal Amiga Emulator that was created for Unix. The Mac has four versions, MacUAE for the Classic, MaxUAE for OSX and the new SDLUAE (Still difficult to install) and E-UAE.

All these emulators need a kickstart file. Kickstart was the Rom of the Amiga. It still has copyright. One can obtain a kickstart by using another Amiga, or by buying Amiga Forever from Cloanto. When one has an old Amiga, it is also legal to search on Google for kick13.zip. MaxUAE has a graphical interface (downloaded seperately), which makes installing easier.

Recently a new version of UAE arose, E-UAE, which has full screen emulation. Together with Hi-Toro (named after the name Amiga inc. had initially), a Gui that even has savable configurations, things are looking up.
E-Uae Homepage
HiToro
The MaxUAE Homepage
The MacUAE Homepage
The sdlUAE Homepage
The winUAE Homepage (Windows XP)