As a computer person, I love this.
Basically, StumbleUpon allows you to download an add-on for your Web browser. You select some of your interests, and you can "stumble" upon sites that you like. You can select whether you like a site or dislike it, and based on what your interests are and what sites you like, you will be able to stumble to sites that other people with similar interests like. The more sites you rank, the more accurate it becomes.
This is fascinating to me because on some level this makes use of a technique called "data mining" - basically, how we can use a computer and some data to make predictions better than any person can do by hand. This is similar in some ways to "wisdom of the crowd" - where if you ask a hundred people who are going to win a football game, the collective opinion of the crowd would tend to be correct.
As with many things I mention here, this is a free Add-on!
- stumbleupon.com (Click on "Get the Add-On")