I have it on my browser toolbar, but I haven't been using Stumbleupon much.  Perhaps I should.  It's apparently much more popular than Twitter, without all the buzz.

It's a way to search the web for info and others who are interested in subjects you search on  and over time it learns what you like - you get to give thumbs up or down and it factors that in when you are hitting the stumble button.

Find out more with this report on ReadWriteWeb that mentions Stumbleupon hitting 7 million users.