Hashcash

Hashcash is a "proof of work" system originally designed as an anti-spam measure. Hashcash tokens are "minted" by finding a hash collision for a unique input string. The result is included in the header of an email message as "proof" that the sender has done a certain amount of work to "pay for" the delivery of the message. A 20-bit collision may take only a fraction of a second to compute, depending on the computer system. 25 bits on the same system would take 11 seconds, and 30 bits would take over 6 minutes. The recipient determines how much work is required to pay for delivery.

The idea behind hashcash is that a legitimate email sender can easily afford a few seconds to mint a stamp for an outgoing email, but spammers who want to send millions of emails would be bogged down.

The hashcash project may be found here.