I've written GUI for few PNG optimizers, which includes optipng
and pngcrush
(get them from the app bundle if you want command-line executables).
BTW: PNGOUT usually gives better compression and has binaries for OS X.
PNGQuant
Moved to pngquant.org. If you want a GUI, then try ImageAlpha, which bundles pngquant and pngnq.