Optimistic Ethereum, while still a very new technology, is up and running, and you can interact with it with MetaMask. For more information on what Optimistic Ethereum is, and why you might want to use it, see their FAQ here.
Adding Optimistic Ethereum to MetaMask
You can manually add Optimism's information to MetaMask (see more information on adding custom networks here) with the following information:
Network Name: Optimistic Ethereum
New RPC URL: https://mainnet.optimism.io
Chain ID: 10
Currency Symbol: ETH
Block Explorer URL: https://optimistic.etherscan.io
For an automated integration, navigate to Chainlink's page for Optimistic Ethereum at https://chainid.link/?network=optimism, click 'connect', and approve the changes in MetaMask.
Note that in order to transfer value into Optimistic Ethereum, you will need to use the Optimistic Ethereum Gateway. Again, Optimistic Ethereum is very new and will continue to change and advance quickly--but at time of writing, withdrawals from Optimistic Ethereum to standard Ethereum mainnet requires a wait time of one week.