libNoise generates coherent noise. Map Maker is similar, but uses a diamond square algorithm. It has not been rigorously tested, but most things that don't involve file reading/writing should work. I really only changed the makefile.
It has also been ported to XNA. A video with libnoise xna.
The quartz composer plugin (Leopard, Tiger) can be useful for trying module compositions without recompiling a program.