Optimism is an aspiring optimistic rollup built on Ethereum that utilizes the EVM. Optimism plans on using fault proofs (i.e. fraud proofs) to secure its blockchain, meaning it assumes all transactions are valid and anyone can issue a proof that challenges if there's something wrong. However, Optimism has disabled its fault proof mechanism and uses a single sequencer node to publish valid state roots to Ethereum. This is so they can push out protocol upgrades much quicker as they don't need to wait for any challenge period (~7 days) to finish.
Optimism supports an ecosystem of DeFi protocols, such as the AMM Velodrome, synthetix issuer Synthetix, perpetual futures protocols Kwenta and Perpetual Protocol, and other well-known Ethereum based protocols like Curve, Aave, and Uniswap.
Optimism is building towards an OP Stack - a modular and open-source blueprint for customizable and interoperable chains.