I ran across Stumble Upon today and I can't say from a brief glance that I totally get it. Looks like one of those that you almost need to experience to get the whole gist of it.
Discover the new world
No more keyword searches - just great videos! Stumble Video automatically finds and plays videos that match your interests. All you have to do is press Stumble! and rate the videos you watch. Stumble Video recommends great videos based on your ratings, and the ratings from other like-minded users. We'll find the best videos on the web and play more for you that match your interests. It's the same great technology that powers the StumbleUpon toolbar used by 1.6 million people.
I guess it sounds a bit like Pandora, except for Videos.