Xaos is a realtime fractal zoomer, which means you can fly around your favorite fractal. I don't know if this is old hat, but xaos does a great job of it, making the rendering and navigation quite seamless.
Xaos can render a number of different fractals, in tons of different styles, including after-effects like edge detection and 3D. It is also scriptable (in a simple ad-hoc way), and includes a tutorial about fractals that shows off lots of cool Xaos features. Also: My kids love it (they call the Mandelbrot set "the apple tush").
The home page is a little discouraging: the latest news entry is dated well over a year ago, and says, "We are alive again" (it reminds me of some half-forgotten Monty Python skit). But the software is great, and is the perfect thing to while away the minutes when you're on hold on the phone.