http://scid.sourceforge.net/download.html
Scid is available for Mac. I don't know what the situation is for engines on a Mac, since I'm not very familiar with Macs, but I'd be very surprised if there weren't good ones available.
Scid takes a lot of getting used to, but it's just a great overall tool for storing and analyzing your own games, working through tactical possibilities, etc etc.
Of course, a Diamond membership is Mac-compatible, and WELL worth it. ;)
Hey,
What is the best piece of chess software for a mac? I've used Sigma, but i don't really like it that much.
Thanks,
Patrick