Discover Magazine has an interesting article on mapping the blogosphere, reporting on the work of Matthew Hurst. The map you see on the top left is the result of that mapping project. Bright spots represent sites with the highest number of links, the isolated islands represent more-or-less closed communities like Livejournal.
The article makes special mention of six groups: the ultra-popular blog sites, gadget hounds, isolated blog communities, socipolitical discourse, porn, and sports.
Read the article for full description of each.