Yup, directory services are a lot easier to do peer-to-peer. Pinboard.in is a good shared directory (sort of Yahoo! without the editorial). They can yield excellent quality when you're searching for something that someone has 'indexed' with them, but poor recall when it comes to the set of all possible answers.

Doing it peer to peer without editorial allows sites to 'get into' the index easily which has its own plusses and minuses.