Bit Gold proposal
The Bit Gold proposal, by Nick Szabo, describes a system for the decentralized creation of unforgeable proof of work chains, with each one being attributed to its discoverer's public key, using timestamps and digital signatures. It is said that these proofs of work would have value because they would be scarce, difficult to produce, could be securely stored and transferred. Szabo's theory of the economics of such money is described in the linked article on the origins of money.
Transfer with prevention of double-spending, via a Byzantine-resilient peer-to-peer method, is described in another linked article which calls the method secure property titles and proposes also applying it to other kinds of digital property, such as domain names. However, this Byzantine method relies on a quorum of network addresses rather than a quorum of (hash) computing power, so unlike bitcoin it is vulnerable to Sybil attacks.