I saw a proposal for spam whitelisting which involved users publishing a hashed list of email addresses. This allows people to receive messages from friends of friends but without revealing your address book. To prevent the data being aggregated, each hash would include your email address in the form:

[email protected]:[email protected]

This prevents spammers aggregating large lists and then rainbow cracking the hashes to find valid addresses. You can also bulk the list with bogus entries to reduce spam and to increase your ego.