JSON input-check от Nikita Matusevich

This is the best extension for validating, formatting and editing of JSON strings.

If you need to validate, format or edit:
* Some single JSON
You can use the popup window. The red indicator in the right-top corner of the input field will show, if you have some errors in JSON string. When you hover over the indicator, it'll show you what exact error you have and in which line. If you see the green indicator in the right-top corner, it means you don't have any errors, but you can hover over this green indicator and click, it will format your JSON string. Also, our JSON editor will help with editing of JSON without any format issues.
* JSON strings in all input fields at your page
You can enable `Check all inputs on the current tab` option at the popup window, and it will inject our extension to all input fields and you'll be able to validate and format JSON input as it's at the popup window.