After the Mandelbrot set, other famous use of fractals is in generating terrain. An early example is Peak from 1980. They are also used in the genesis sequence in Star Trek II: The Wrath of Khan
Fractals have not just found use in generating pretty pictures. The Space filling curves mentioned earlier have found a use in creating compact aerials in mobile devices. Iterated function systems like the previously mentioned Lindenmayer systems have been used for image compression but due to some early patents work has slowed because of the legal risks. Modelling financial markets is another application, originally proposed by Benoît Mandelbrot , which over the last few years has been enjoying renewed interest.
Rendering of fractal landscapes has also not stood still, with the power of modern hardware, much more realistic terrain can now be rendered in real-time instead of to a film recorder as in the past, producing result like, Elevated by RGBA & TBC, a 4 kilobyte demo running on a normal PC.
For any technical people, one of the programmers gave a presentation on how the terrain is generated and rendered (PDF) if you are interested.
I have more links on the fractals tag on pinboard is you want to explore some more.