blogcosmos
Edd Dumbill explains the whole Planet project aggregator thing in the course of introducing Planet RDF. Edd calls this "bring[ing] together the journals of people working on the same project into one site," and it's that, but it's also another way to create topic centers on the Web, a place to go for aggregated thought and expertise on specific subjects - right now it's mostly open source geeks, but I could see how this could spread much further. Of course it needs to come with the same caveat lector you get anywhere on the web.