According to Hayden Adams in Uniswap V2 launch blogpost, Uniswap V2 has been deployed to the Ethereum mainnet.
What is Uniswap?
Uniswap is a decentralized Ethereum based exchange that allows you to do ERC20 swaps, ERC20 to ETH swaps and ETH to ERC20 swaps.
Who invented Uniswap?
Hayden Adams is the inventor of the Uniswap Protocol.
How to get access to the Uniswap Protocol?
The Uniswap Protocol can be accessed through its front-end : https://uniswap.exchange/swap

Who can swap tokens using Uniswap?
Anyone with an Ethereum wallet listed when selecting "Connect Wallet" can swap tokens.

Can I use MEW (MyEtherWallet)?
Yes you can use MEW after connecting your MEW Wallet to MetaMask. You can read my article "Connect your MEW wallet to MetaMask" to learn how to do it.
What about Uniswap V1?
Uniswap V1 will continue functioning on Ethereum.