From the StumbleUpon home page:
StumbleUpon is a completely new way to surf the web. It lets you channel-surf pages which your friends and peers recommend.
Available for Windows, Macintosh & Linux
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.
From their "What is it?" page:
"StumbleUpon is a free browser extension which lets you channelsurf the web. It acts as an intelligent browsing tool for exchanging websites with friends and like-minded people.
StumbleUpon uses member ratings to form collaborative opinions on web site quality – you'll see only the sites that friends and like-minded stumblers have suggested. Unlike search engines or static directories it allows for a true "democracy of the web" – everyone has a say in which sites should be passed on."
