A good 3D fractal engine?

I’m looking for a good 3D (pseudo-) fractal engine; VC-source or theory, anything that helps me. I’m thinking about a serious one, and I am especially interested in those with advanced and flexible tree modeling, or whatever has to do with nature. I also would like to get in touch with people who like to model and fiddle with fractals.

Can you help me? thanx in advance

There are some good resources at http://www.vterrain.org