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

    The hashcash tool allows you to create hashcash tokens to attach to
    emails you send, and to verify hashcash tokens attached to emails you
    receive. Email senders attach hashcash tokens with the X-Hashcash:
    header. Vendors and authors of anti-spam tools are encouraged to exempt
    mail sent with hashcash from their blacklists and content based
    filtering rules.

Homepage:
http://www.hashcash.org/
