One option is to use the landscape generation script with blender. This has some dis-advantages because blender really isn't designed to do the landscape generation thing, so the results won't be as good as terragen (a mesh modeller doesn't so it the same way as a fractal landscape generator, so achieving the same level of detail isn't easy). Anyhow, here's a link
Although it's a bit of a cop-out, you could also try running Terragen under wine. I had some success with this approach. I did a write-up about this, which you can find here
. It's not 100%. Some important options (like saving terrain files) don't work. Still, it's fun to play with.