Kashmir 3D
http://www.kashmir3d.com/index-e.html
For those who like Terragen (Ihaven’t used either one).
In my honest opinion… Terragen is much better.
Kashmir was what got me started with Terragen. I though it was a cool toy (and it had lots of good surface terrain presets), but Terragen has many more features and many more ways to customize. When it comes down to it, Kashmir makes pictures, Terragen makes art (well, when you use it right 
).