BoycottAdvance/MacOS is a Game Boy Advance emulator for the Macintosh. It can run most of the existing Game Boy Advance software to some degree, however somewhat slowly. A PowerPC G4/400 is the realistic minimum system, and even then you will not get full speed all the time.
The emulator supports full and accurate sound emulation – most games sound perfect, although a few suffer from a slight clicking noise. Boycott Advance is not a substitute for the real thing, however. If you really want to play Game Boy Advance games, then buy a real Game Boy Advance.
More info and download link here.