Using their interface....you click on the stumble button and it randomly pulls up pages, I think that the other users have added to the list of sites.
I mention it because a lot of the sites have been really cool.
The design ones have been quite compelling, for example:
http://www.zerohouse.net/ When I can afford it, we are getting one and moving in.
http://yugop.com/ this one is just a really great design site with great interaction.
They have a toolbar that can be added to your browser so you can use the stumble option any time.