ZCoin is a privacy focused cryptocurrency built on Ethereum. The ZCoin protocol enables anonymous transactions with cryptography that utilizes Zero-Knowledge Proofs (ZKP). ZCoin increases privacy on their network by using a coin-mixer contract that wraps transactions in a serial number and destroying wrapped coins once they have been redeemed.
Zcoin was the first cryptocurrency to use a Merkle Tree Proof (MTP) as its proof of work algorithm. Merkle Tree Proofs are an innovative solution to achieving network consensus, and can increase transaction time while reducing the centralizing tendency of ASIC mining.

BurstCoin is a cryptocurrency with a Proof-of-Capacity (PoC) consensus algorithm. Unlike most other cryptocurrencies, Burst's architecture uses a Tangle (similar to IOTA) instead of relying on a blockchain. It also utilizes ring signatures and other features, in what is called the Dymaxion. The Dymaxion implements layers of tangle-based lightning networks on top of the Burst blockchain. Each of these payment channels is opened in the form of a standalone colored tangle. Burst is also developing atomic cross-chain transactions (ACCT), that will allow for full decentralized trading between two cryptocurrencies.

BitcoinDark is a fork of Bitcoin with a poorly defined purpose and use case. BitcoinDark is based on the InstantDEX protocol, a decentralized exchange for BitcoinDark and other cryptocurrencies supported by the project. The platform is also supposed to include a Pegged Asset Exchange (PAX) to allow users to buy and sell pegged assets – including fiat currencies, commodities, ETFs, and stocks.
BitcoinDark is a Proof-of-Stake (PoS) currency that has gained a significant market cap, but the project has failed to show how its vision will be implemented or how it will add value compared to other blockchain projects.