Arbitrum is a layer-2 scaling solution for Ethereum developed by OffChain Labs. It is an Optimistic Rollup on top of Ethereum, which allows for lower fees and faster transactions. Arbitrum went live on mainnet on August of 2021.
Main protocols
Arbitrum's main protocols are DEXs like Uniswap, Sushiswap, Balancer and Curve , bridges like Synapse and Stargate and money markets such as Aave. Derivatives trading protocols have also accrued substantial TVL, like Dopex and GMX