XaoS is free and open source, and runs on a variety of operating systems (Windows, Mac OS X, Linux, BSD, DOS and more). You can get it from here.
It is fast - but so are many others.
What XaoS is really good at is zooming into the Mandelbrot fractal so fluidly that the effect is an animation of you fluidly diving deeper and deeper.into the fractal with detail appearing without a glitch. See for yourself with the following videos.
This is actually amazing, given how computationally intensive it is to calculate just one frame of such a video. I recall how in the 1980s it took hours on a home computer to calculate even a low resolution single frame.