Email Stamps

CentMail is new attempt to fight spam by using digital stamps.It is backed by Yahoo research. The proceedings from centmail go to a charity chosen by the sender.
It is well known that even such a small cost for sending out spam will kill the business modell of most spammers. However, it remains to be seen how email receivers will react to these stamps. I would consider acknowledging them in my spamassassin rules.
Also note that people still need to guard against malware received by email even if such a system becomes popular, as a targeted malware infection may have a completely different business model.
Anyway, the money is well spent even if it doesn't kill spam immediately.