Have you heard of Terragen?

It’s software for generating real looking landscapes. This is not an actual mountain, but a generated image.

More terragen images by artist Luc Bianco.

Terragen™ still offers a free beta version if you want to try it.

George recommends these sites:

http://www.digitalblasphemy.com/userg/user1.shtml
http://swaroop.deviantart.com/gallery/
http://www.begann.de/start.html
http://www.terradreams.de/
http://gallery.artofgregmartin.com/wallpapers/
http://www.inganielsen.de.tc/
http://www.moodflow.com/