Recently, I've heard a lot about the L'il Miss' Scale Oven™ microtonal software. It sounds like a great resource for microtonal musicians.
Alas, I can't comment on it personally as it's only for Mac, and I'm a PC user. I did, however, run across a glowing review of it at the Post Classic blog. You can also visit the Nonoctave.com website for ordering information.
Even if you use a PC or aren't interested in the software, it is worth visiting this site because it is a good source of information on nonoctave music, an interesting subcategory of microtonal music.
If you have used L'il Miss' Scale Oven™, please post a comment telling us what you think. If you have any comments on other Mac only tuning software, that would also be appreciated.
See also the Bohlen-Pierce Site, another good web site on nonoctave music.