Echidna is a weird creature that eats bugs and is highly electrosensitive (with apologies to Jacob Stanley) More seriously, Echidna is a Haskell program designed for fuzzing/property-based testing of Ethereum smart contracts. It uses sophisticated grammar-based fuzzing campaigns based on a contract ABI to falsify user-defined predicates or Solidity assertions.

This is not a comprehensive list and solidity is quickly evolving so please do due dilegence on your part. Not all listed items will apply to your specific smart contract.