World Builder creates random world maps, as a useful starting point for creating conworlds. It is actually a front-end to a command-line program (also included) called planet.exe, developed by Torben Mogensen.
For an overview of the program and its features, check out the on-line help for it.
There are two download options. The full option includes all the help files and installs them on your computer along with the main program. The smaller version does not include the help files, and the program will instead send you to the copy of the help files on this site.
You can also check out screen shots of the program here.