UltraFractal is a shareware program with a high degree of compatibility with Fractint, but with "true color", use of the alpha transparency channel, and Photoshop type layering capabilities. There is no other program around like it.

Fractint. This freeware program, which generates fractals, is free, still the fastest, and one of the first.

Apophysis is a freeware program developed by Mark Townsend to create IFS images. The parameters can be exported to UltraFractal for further processing.

POVRay is a freeware ray tracing program which can do virtually anything the expensive commercial programs can do, and often much more. Try it, you will like it.

If you want to generate trees, bushes and many other interesting objects, the freeware l-system program to look at is Lparser.