Stumble Upon has some people moving power.
Simple Curiosity? my 'irrelevant thoughts blog' has a post on it that must have been picked up by Stumble Upon the day before yesterday because when I logged in, I saw that I had an extra 5,000 visitors to a post I wrote on Why time slows down when approaching the Speed of Light. Bada bing.
I'm not sure of the long term benefits of being stumbled but it did add another 150 rss subscribers so I'm nothing but happy.