It is a toolbar which allows you to stumble through the Internet and every time you rate a website with a „thumb up“ or „thumb down“ the toolbar learns to filter that kind of websites you like, recommending those instead of pages you don’t like.
The first time I heard about this nice, sometimes time consuming, toolbar was at the Mozilla Firefox Extensions website. After registering for free, since they have to store your „thumb ups/downs“, I used it the first time…and got addicted. I believe I „stumbled“ about an hour straight, then I decided to take a break.
Today I „stumble“ whenever I’m bored or looking for something interesting and fortunately I’m not as much addicted anymore like I was the first days I got this extension.
I really recommend you get the StumbleUpon toolbar. (and don’t forget to give me a „Thumb up“ 😉 )
It is free! It is fun! It is StumbleUpon!