I was just in the processing of setting up some PubSub feeds for internal purposes when I saw their new LinkRanks feature. Wow. Very, very interesting (you can also read the about complete with mathematical formulas.
What does it do? It tracks the popularity of links to domains (domains, not individual pages). It specifically only looks at links that are within the body of syndicated content (which is pretty much limited to weblogs at this point). So, all those linkrolls and sidebars don't count.
Also, if you include a link like this:
http://psi.pubsub.com/20040413:linkranks:1
Then PubSub will track the "conversation" around their LinkRanks topic. This is another feature which doesn't look to be "live" yet, but seems very, very interesting.
The potential here is to have true communication between and around web pages and blogs -- by pointing to a common link, all posts around a particular topic could be gathered together in one place.