StumbleUpon, the web surfing social network, is a brilliant toolbar that embeds into your browser and allows you the channel surf through thousands of pages, blogs, sites and tags that have been stumbled upon by other web-users."
StumbleUpon helps you discover and share great websites. As you click Stumble!, you’re delivered high-quality pages matched to your personal preferences. These pages have been explicitly recommended by your friends or one of 2,148,327 other websurfers with interests similar to you. Rating these sites you like () automatically shares them with like-minded people – and helps you discover great sites your friends recommend.
StumbleUpon uses / ratings to form collaborative opinions on website quality. When you stumble, you will only see pages which friends and like-minded stumblers () have recommended. This helps you discover great content you probably wouldn't find using a search engine.