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Below is an list of Ethereum related second-layer side-chain designs which are currently undergoing development on GitHub.
Cosmos, Peg Zone Implementation
Taiwan Team, Javascript implementation of Plasma MVP
Blockchain at Berkeley, Plasma Debit
OmiseGO Plasma Cash - GitHub Repository
OmiseGO Plasma MVP - GitHub Repository
Please note: The list is in alphabetical order (based on the GitHub account which houses the code)