I know Incog has posted about Bryce, I thought I'd share with those that are interested in free alternatives.
http://www.blender3d.org/
^^ This is a superb bit of freeware, just one look in the image gallery will show you how powerful and versatile it is.
I don't know how easy it is to use, as I haven't yet tested it myself, but judging by the results others have achieved it can't be too bad.
I'll be posting my first ever render attempt in here.. post yours if you have a go yourself.
( make sure you get this installed too:
http://www.blender3d.org/cms/Yafray.51.0.html .. read the notes on the site to see how to use the two together )