I use Apophysis,Fractal Explorer and sometimes Ultrafractal,but there are many more fractal apps. What program to use is up to you,give some of them a try and look wich one gives you a good feeling and a result you like. Check out the fractal gallery also to see what you can do with the different fractal applications. That should give you a good idea what program to use according to what you want. Good luck.