I've been playing with 2 fractal-generating applications recently, and recommend them for different reasons. If you'd like to quickly generate random beautiful 2d images, of often breath-taking beauty, use
Apophysis.
If you like to play with dials and sliders and 3d imagery, and generally do a bit more work yourself, I recommend Chaoscope, a "3d strange attractors" rendering package.
Samples from both: