Terragen

Terragen software is capable of photorealistic results for professional landscape visualisation, special effects, art and recreation. Terragen is a work-in-progress scenery generator for Windows and the Mac OS. At this stage of its development, Terragen is free for personal non-commercial use.

This is very cool software. It generates absolutely beautiful pictures. It appears to be very easy to use. The results are startling.

http://www.planetside.co.uk/terragen/