One of the best and most widely used is 'UltraFractal', which gives a lot of control over colors and allows interactions between layers similar to Photoshop. UF3 is commercial, but moderately priced. Recently I tried about half a dozen freeware programs before purchasing UF3, and my favorites are 'vchira' and 'apophysis', which are complementary in the types of fractals they generate. [my OS is Windows; I don't know about availability for Macintosh]