Hashcash
"The Hashcash proof-of-work function was invented in 1997 by Adam Back, and proposed for anti-DoS uses including preventing: anonymous remailer and mail2news gateway abuse, nym name squatting on nymservers (replyable pseudonymous remailer severs), as well as general email anti-spam and general network abuse throttling. Before bitcoin, hashcash was used by SpamAssasin, and (with an incompatible format) by Microsoft (with the name "email postmark") in hotmail, exchange, outlook etc and by i2p anonymity network, mixmaster anonymous remailer components and other systems. Hashcash was also used by Hal Finney's bitcoin precursor RPOW as a way to mine coins. Wei Dai's B-money Proposal, and Nick Szabo's similar Bit Gold proposal bitcoin precursors, also were proposed in the context of hashcash mining."
