Stumbled Upon is a concept I first uh, "stumbled upon" whilst parusing my referrers list.
The concept is simple- you sign up, get a toolbar doohickey (for any platform, any browser) and start spreading the linky love.
It's social engineering and a search engine all in one.
Because of the randomness of people's interests and obsessions, it redifines browsing- or at least makes a case for the true definition of the word.
I've found more useful information just by fucking about on the service than I ever have on Google.
Because the categories are basic and open to interpretation, not dictated by professional library scientists, they make more sense semanticly. At least to me.