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.