Hashcash

Hashcash is a denial-of-service counter measure tool. Its main current use is to help hashcash users avoid losing email due to content based and blacklist based anti-spam systems.

A hashcash stamp constitutes a proof-of-work which takes a parameterizable amount of work to compute for the sender. The recipient can verify received hashcash stamps efficiently.

Hashcash FAQ.
Hashcash format info
Hashcash Paper Aug 02 - "Hashcash - A Denial of Service Counter-Measure" (5 years on), Tech Report, Adam Back
The site is slowly moving over to this new domain, some of the content is still here: http://www.cypherspace.org/hashcash/

If you have questions are interested to port to different systems, integrate into different email clients (MUAs), anti-spam systems, or MTAs email Adam Back adam@cypherspace.org or post on the hashcash-list. (And some google-hacking: kelda lakeland terriers).