What do you call Stumbleupon? Its developer calls it a discovery engine, and that is pretty much what it is. Users are encouraged to stumble upon content on the internet that they perhaps might not have otherwise seen, and share or recommend it to their friends.
It is a great time killer, letting as it does flick through the uncharted internet, and also serves as an educational tool as it takes its users through new and fresh content.
You can look at what other people have enjoyed, based on a set of topic and theme parameters that you set yourself, and even helps users discover other great applications based on their own tastes.