I typically use PovRay (www.povray.org) to render the landscapes I generate. A while ago though, on www.slashdot.org there was mention of a Windows (blah!) application called 'terragen' which produces photorealistic output. I haven't downloaded/installed it but the web page they have looks very nice. Of course, in the wonderful world on MS Windows, they don't seem to have any intention of releasing the source.
If the lack raytraced output annoys you, I invite you to respond to my Call to Arms below.