Beertools Pro seems to be the most popular brewing software for Mac platforms (although I have only ever tried the Windows version).

The overall most popular software package out there is probably Beersmith. It is Windows-only software, but on the Beersmith forum there are reports that it works well on a Mac using a Windows emulator.

There are also a couple of freeware, Java-based, cross-platform programs worth checking out: Qbrew and StrangeBrew Java. They should run on any platform that supports a Java engine, although these packages aren't nearly as slick as the commercial ones.