Redux DevTools by timdorr, Methuselah96

Redux DevTools is a development time package that provides power-ups for your Redux development workflow.

• Lets you inspect every state and action payload.
• Lets you go back in time by “cancelling” actions.
• If you change the reducer code, each “staged” action will be re-evaluated.
• If the reducers throw, you will see during which action this happened, and what the error was.

It's an opensource project. See the official repository for more details.