StumbleUpon is a website and content discovery service enabled via a browser toolbar. StumbleUpon uses positive and negative user ratings to form collaborative opinions on website quality. When users stumble, they will only see pages which friends and like-minded stumblers have recommended. More often than not, it's something almost serendipitously interesting to the reader. The company expanded into video discovery in late 2006.
People who are passionate about StumbleUpon say they like it because of the surprise factor in what they see next, and the fact that the product has such a high hit rate in delivering interesting new content. The StumbleUpon site says they have 2.1 million users, up from 1.7 million in December 2006. 4+ million sites are "stumbled" daily.
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Very nice.... Stumble upon is very addicting... thanks!