Flipper Developer

Flipper Developer is a wallet that allows developers to create and test smart contracts in a production-like environment before making them available to the public.

If you're looking for the Flipper wallet, which supports actual-value transactions on the public network, you may find it here:
https://chrome.google.com/webstore/detail/flipper/lmmpaefggfcmnoddemmgdppddppnmhek