StumbleUpon is a fiendishly simple concept executed flawlessly.
To grok the concept, imagine all of the websites in all of the world are laid out on a big dartboard in their normal mixed-up state. Everything.
StumbleUpon hands you darts that are designed to hit just the stuff you're interested in. Exactly what sort of stuff, you don't know. That's why you throw the dart.
Or in this case, you hit the Stumble button that StumbleUpon installs in a neat little bar in your browser. And voila, you get something unexpected. In the few hours I've spent playing with it, I'm astounded at how well it work.
Then you decide whether you like what you've hit, or not. Or frankly, you don't have to do anything. Unless you want to throw another dart (hit the Stumble button again, that is).
What you hit depends on what sorts of things you've decided to tell StumbleUpon that you like; StumbleUpon then uses an index of websites that other users have tagged to decide what to show you.
You Stumble, you give a site a thumbs up or down, maybe even leave a comment if you like, then you Stumble again.
The fiendishly simple concept is preference-based guided browsing. The index gets smarter and bigger based on the size of the user base - more people bring more sites and more opinions. It's still all about browsing - but it manages to bring a sense of community via a very rich personal profiling system that automagically shows you how your interests align with those of people who like the same sorts of sites you do.
When you consider the broad range of subject areas StumbleUpon exposes as preference areas, it allows users ways to define themselves in equally broad terms. And that's doesn't include your personal preference areas, which reveal your connections to other users.
I installed it into my Firefox browser earlier this evening, and I'm already wearing out the Stumble button.
It makes the "I Feel Lucky" button in Google seem stupid. Because it is. I dont' want to see random s--t, I want to see random s--t I'm actually interested in. And there's the difference.
If you've been trying to find a way to get past the 90% of the internet that's crap to the 10% that rocks, install StumbleUpon immediately.
(Edit - I love how they position their value-add stuff - you "sponsor" StumbleUpon at a level that makes sense to you, kind of like National Public Radio. Heaven knows this can't be the only way they plan to make money, but it's a beautiful way to reinforce the sense that you're part of the StumbleUpon community. Nicely done.)