Back in 2006, Google had a product named related links, which showed related sites on the internet based on your content. Google just recently relaunched related links at relatedlinks.googlelabs.com. The major difference between the old related links and the new one is that the new one only shows related content from within your site, whereas the old one took away visitors from your site.
The new related links can be tried out over here. Here is a picture of how they may look:
Back in the day, Yahoo had a product that was somewhat similar named YQ. YQ is no longer active at this point.