So many worlds, so little time ...
About TerraJTerraJ is a set of fractal terrain and solar system generation programs that have been ported from C/C++ to Java.
The port has included extensive refactoring, the creation of a documented API to enable reuse of parts of them in other programs and extensions to the original programs such as new Swing user interfaces. Full javadoc for all the classes in the project is available.
Example images from the programs are shown below. Click for full-size versions: