Social: 
LOAF
is ‘a way to share your address book without abandoning your privacy.’
A nifty use of Bloom filters to share your address book in a one-way manner — when you receive a mail, you can query your LOAF db to see if any of your correspondents previously corresponded with the sender; but they cannot look up the LOAF file to determine your correspondents, unless they know that correspondent’s email address in advance.
This, BTW, would be a very good way to implement a ‘Do-Not-Email’ list — although the other two problems with those still apply.
Interesting stuff — although I wonder how acceptable the 4-8Kb MIME part overhead per message will be…