ZCoin is a Proof of Work cryptocurrency using the Lyra2 hashing algorithm. It is an implementation of the Zerocoin protocol (http://zerocoin.org) guaranteeing true financial anonymity using ZK-Snarks. Zero-Knowledge proofs allows one to show ownership of a Zcoin coin without having to reveal which coin one owns.
Recently, the Zcoin protocol was found to have a typographic bug that costed the network roughly $400k. This bug, however, did not compromise the anonimity features of Zcoin.