FirePay is an implementation of the HashCash/MIME 
(application/postage-hashcash) specification. HashCash is an anti-spam,
anti-DoS system for email. The concept centers around the sender of the 
mail completing a computationally expensive task (in this case, 
generating hash collisions) that the recipient can verify very quickly.
This allows the sender to "pay" for transport in CPU cycles, creating 
artificial scarcity. Programs are included to add payment to messages 
(for the sender) and to verify message payment (for the recipient).