Mapgen is a project mainly written in C++, it's free.
Johan Halmén's fractal map generator
Note: The docs folder is old, this whole project hasn't been touched in years until it went up on github. But read it anyway, it has some relevant information.
== New version WIP == I have started rebuilding the generator from scratch. I'm using C++ and Allegro 5 and render the map directly to 3D with OpenGL. I'm using mtrand instead of the crappy standard rand() because people were complaining in IRC.
=== Building === There's now a premake4 build script.
==== Old mapgen ==== The old mapgen builds using Allegro 4.4. The project is commented out in the script, uncomment it to build. The exe is build/mapgen
==== New Mapgen ==== This depends on a 3D library I made called Alledge. https://github.com/trezker/alledge No GUI yet, but when I add it I'll probably use SWAG. https://github.com/trezker/SWAG Exe is build/mapgen5
=== 3D view === Uses Allegros old software 3D routines. It doesn't matter how good your gfx hardware is. But it runs nicely anyway on a modern computer.