Its sort of a social bookmarking/sharing website where people "stumble" websites they find interesting and people who are bored end up getting sent to it. Check it out at Stumbleupon.com
Basically, a toolbar. You mark sites you like with a thumbs up, and ones you don't with a thumbs down. You also have a Stumble! button which leads you to a random liked page within your interests. Pretty good, I'd join. Nice source of traffic!
stumble upon is a social bookmarking sites that let you share your favorite websites to your friends and around the world who is using this social book markings. it is also used to bookmarks your favorites and other stuff
Hmm...you could have just looked round the forum a bit, as this question was asked and answered yesterday. http://forums.digitalpoint.com/showthread.php?t=567620