I use to use this application for fun a couple of years back when i was in high school and could not get my hands on a good 3D app, there stiill is a free verison out there
http://www.planetside.co.uk/terragen/
not sure now much less you get in the free version or if it has even been updated in a while, I rememeber the render times were not to horrendous even on my old windows 98 box
just thought if anyone want to try it out for fun i would give the link to the free version, its not very intuitive or at least when I was using it, it wasnt.
Cheers,
Pat