One of the 3D fractals that I like very much is the Menger sponge.
As the name suggests, it was discovered by Sir Karl Menger at the end of the twenties.
The implementation of this fractal has more things in common with the IFS (iterated function system) than with the Mandelbrot formulas.
At the bottom let me show you some original pictures that were inspired by the Menger Sponge.
Some of the pictures have a fractal background created with JWildfire.
For instance the #2 is a collaboration with another artist (she created the background image).
Later all layers were edited with Gimp.
License
Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)
You can watch some of my work at 500px