Penny Post is a system that helps combat spam emails. Using Penny Post, the sender "pays for email" by spending a few seconds of computing time. Outgoing mail is automatically marked with postage stamps that convince the receiver that the sender is genuine.
The Thunderbird extension makes it a breeze to stamp email and verify stamps, plus it looks cool. Try it!
The latest release for Thunderbird is Vista compatible and has minor bug-fixes. It also ships with Penny Post v1.2. An update is recommended for all existing users.