I use Apophysis, you can also use Tierazon, Sterlingware, Fractal Explorer which you download all of those for free or like Ultra Fractal you would have to pay for. There are more but those are the ones I know of.
I use Teirazon, Sterlingware, Ink Blot, Kazta, Vchira, Flariun 24, Fractal Explorer, MBF Lite and Chaoscope.. All of these are free downloads.. Links to some of these can be found in the FAQ's
Tierazon, Chaoscope, ChaosPro, VChira, Fractal Explorer - all of which you can download for free. Great fun. Great programs. Then I use a graphics program to enhance them further - my favorite being Photoshop.