When Satoshi Nakamoto released the Bitcoin whitepaper, he proved that the double-spending problem could be solved through a distributed payment network. Since then, there has been a large body of digital currency research which allows for significant improvements to be made on top of Bitcoin.
Zcoin has implemented the Zerocoin Protocol, one of the most groundbreaking academic developments in Bitcoin research. The Zerocoin Protocol uses Zero-Knowledge proofs to guarantee complete financial privacy and anonymity. The original Zerocoin paper can be found here