StumbleUpon is a new way to surf the web. It lets you channel-surf pages recommended by friends and peers – great sites you can't find using search engines.
We are a community-based, word-of-mouth approach to websurfing - pages you "stumble upon" come from like-minded people who share your interests. Add the Toolbar, choose some topics and click Stumble! You'll meet people who like your favorite sites as you discover the best of the web.
Basically you tell it what sort of sites you are interested in, and it comes up with random pages based on your preferences. By encouraging people to rate the sites a natural evolution of "survival of the most interesting occurs."