I've just rediscovered the joys of the simple God simulator (or plate tectonics for you Evolutionists!) Terragen.

It's a free download:

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

and for some really superb professional creation, see the results here:

http://begann.de/startbig.html

I would dearly love terragen.. if only I were any good at actually designing landscapes!

Im waiting for version 2 to come out for use in CG environments. Current one is pretty cool, bot does not provide the detail i need.