HP Labs PDF on analyzing communities by tracking email, making a huge connected graph of that data, and breaking into communities by removing links that have high 'betweenness' (that are traversed by many shortest paths).
The Circle is a decentralized hashtable and the applications that use it. It currently provides file sharing, messenging / chat, and trust-based news. Fascinating. Also has a link to the Rational Street Performer Protocol, which is a way of raising money for ongoing public works in which you gain more utility from contributing than from freeloading. MemeStreams separates journal-style weblogging from link-propagating weblogging, lets users easily share links, monitors clickthroughs and subsequent blogging to do collaborative filtering and customize searches for each user, and has categorization and user groups. Fascinating. Scalable? Useful beyond Blogdex for now, although if everyone used RDF (say, RSS 1.0) eventually you could get your client to do reputation stuff (even with a distributed search) and customize in the same way.
Online dictionary translates among Ukrainian, Russian, Bilorussian, Polish, and English. Very cool, if you ask me.