Adam Back proposes Hashcash, a proof-of-work system originally designed to limit email spam and denial-of-service attacks. The concept required a sender to perform a computational task before sending a message, making mass spam economically impractical.
Hashcash's proof-of-work mechanism became a direct ancestor of Bitcoin's mining algorithm. Satoshi Nakamoto cited Hashcash in the Bitcoin whitepaper, and Adam Back was one of the first people Satoshi contacted before publishing the paper.
Read the Hashcash paper