Clay Shirky has posted a summary of LOAF, a perl app designed to create networks of trust among email correspondents. The email addresses are obscured by a Bloom Filter in order to prevent "Ex-Girlfriend" attacks :)

LOAF works by attaching a small file to each outgoing email you send, and monitoring your incoming email for LOAF attachments. These attachments are automatically stored away in one of two folders - a 'trusted' area for people you have corresponded with before, and a 'strangers' area for people who are unknown to you.

Each incoming mail is tested against the LOAF files of all the people in your trusted folder. If the sender has corresponded with one of your friends before, a header is added to the email to indicate the number of matches, and the addresses that matched. Source