Check it out here.

Pretty nice, but not cheap. If I had the money, I'd probably have this, Leveller, L3DT, FractalTerrains and GeoControl on my computer. As it is, I rely on Wilbur, QGIS, GRASS, Photoshop, Image Magick, GDAL and Inkscape. Photoshop is expensive(as hell), but I already had a cheap copy from my school daze.

For the most part, the GIS tools aren't well designed for generation of imaginary geography, but with a little skill, a bit of imagination and a huge willingness to bend the analysis tools in ways they were never intended you can do wonders.