Bitcoin Cash (ticker symbol: BCH) is a decentralized uncensorable peer-to-peer electronic cash system as outlined in the Bitcoin whitepaper by Satoshi Nakamoto that has fast, secure, reliable, and low fee transactions running on a permissionless blockchain network. Bitcoin Cash is the upgraded version of Bitcoin which carries the same genesis block that was started with Bitcoin in January 2009. You can learn more about it by reading What is Bitcoin Cash.
BCH doesn't have a single official Github repository. Due to it's decentralized blockchain and p2p network, there are several Bitcoin Cash software client implementations, listed below in no specific order.
For a more comprehensive list of other BCH projects that use Bitcoin Cash, visit the Bitcoin Cash Projects list on Bitcoin.com to see what kind of applications and tools are being built.