Redux DevTools v3.1.6 

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.