I can recommend the following software:
E-UAE:
http://www.rcdrummond.net/uae/
this requires an OS X version of...
LibSDL:
http://www.libsdl.org/download-1.2.php
And to setup a .uaerc (bootstrap config file) I recommend...
HiToro:
http://www.pimley.net/projects_hi-toro.html
It takes a little futzing around to get working, the current version of E-UAE is only compiled for PPC (runs via Rosetta on Intel) and overall E-UAE is not as fast as WinUAE. It's no slouch on decent PPC hardware though, it ran at a decent enough speed on my Powerbook G4/667. On Intel I get better results from WinUAE running through Parallels.
In order to complete amiga emulation you need a legal copy of one of the Amiga ROM images. I recommend having at least a 1.3 and 3.1 ROM for compatibility reasons. I strongly advise you purchase AmigaForever by Cloanto, available at:
http://www.amigaforever.com/
AmigaForever contains not only legal ROM images from Amiga ROMS, with an unlock key (that is supported by all major UAE releases), but it also includes many licensed Amiga software packages, and also a license for Amiga OS 3.1.
(Disclosure: I have no connection to Cloanto, but I have been using AmigaForever, and recommending it as a secretary of an Amiga user group, for 3 years)
I do not condone or encourage illegal ripping or downloading of ROMs from any older hardware, Mac, Amiga or otherwise.