I just discovered mandelbulber by Krzysztof Marczak (buddhi1980), a frequent visitor of Fractalforum. It is an open source program to ray-trace 3D fractals, and in my opinion this is simply awesome. It can be used to render images or even fly-through movies. The program can be found as project mandelbulber on Sourceforge.
See this movie for example, the amount of detail is incredible.
It makes me wonder if this would be possible in real-time, it would be incredible to fly through such a contraption. What would it take? GPU? CUDA? Amazon EC2?