Blog Big Picture gives an overview of recent activity on several sets of blogs by showing you which blog entries mention what. When you click on a category on the right, it lists terms from that category (for example, in the hollywood set of blogs, clicking on Movie lists specific movies mentioned in the blog entries in the main panel), and then passing your mouse over those movie titles highlights both the movie titles and the titles of the blog entries that mention any given movie. Clicking any blog entry title takes you to the blog entry.
The categories of blogs on the various tabs were chosen for demo purposes. Many other categories would work well.
Identification of terms in the blog entries is done using OpenCalais, and assembly and sorting of the identified terms and other blog information is done using RDF technology. So it's a semantic web thing.