Using fractals to generate planetary imagery can aid worldbuilder writers who, like me, have to create whole worlds from the core out. Ed Hynan's Fractal Planet Generator is my personal favorite, but Felix Golubov has come up with a pretty neat little program for planets and one for landscapes, too.
John Olsson's Fractal WorldMap Generator is another one I use frequently, and presents the image in a number of formats, including mercator and spherical.
Jim Burrows has one for Mac users here. (I can't test drive this one, don't have a Mac.)
Not enough? Try one of the many fractal programs listed over at Paul N. Lee's Fractal Software Links Page.