Called SimilarWeb, this small Firefox (and soon Internet Explorer) add-on sits on the side of your browser and pulls up sites that are similar to the one you're currently on. For example, visiting YouTube brings up a long list of other video hosts. The same went for the social news site like Digg.
You can scroll through these and open them up in new tabs, or pick from one of the tags SimilarWeb believes to be related to that page. This will pull up an entirely new list of places it thinks you should visit.
Users can also re-arrange the lists and submit new sites that are not yet in SimilarWeb's index. There are thumbs up and down buttons which can raise or lower a site's standing on the list. Down-voting any site will actually remove it from the list. As a result, if users continue to vote the list gets more accurate.
I really like SimilarWeb and it reminds me of StumbleUpon which learns from the sites you browse and suggests similar sites. So, between the two sites, I have come across some very fascinating websites.