Been playing with a free 3D fractal rendering tool called Mandelbulber. The interface is a bit spartan but it can create some really sweet renders. I just played around with the example fractal files already included with the software as I don’t really comprehend the maths behind it. Post work done in Photoshop to make them look a bit prettier.
You can download Mandelbulber here: http://www.mandelbulber.com/
Found out about Mandelbulber from this amazing website: http://chaoticatmospheres.com/. The stuff on that site is stellar. Hope to try the Cinema 4D attractor plugin he talks about sometime as well.
EDIT: Also, don’t forget the Atari/Lucasfilm game from the 80s starring the first 3D-ish environment using fractal mountains! http://en.wikipedia.org/wiki/Rescue_on_Fractalus!
Allow me to geek out over Rescue on Fractalus for a moment. ILM made a model of the fictional spacecraft just for the box even though you never see it from the outside in the game: http://superadventuresingaming.blogspot.com/2013/04/rescue-on-fractalus-atari-8-bit.html