I've been spending some time learning to do oblique terrain work in Blender, a free 3D modeling program. I've previously used Natural Scene Designer for this sort of thing, but I've not been satisfied by some of its limitations as far as making good-looking, semi-realistic land cover. I'm no Blender expert, but I feel like I'm starting to get the hang of it.

It’s not something I get a lot of call for — it’s really only useful in certain special cases, but it’s pretty, and it’s fun to play with.