JSON Formatter - makes raw JSON easy to read with collapsable trees

LiveReload

Postman - REST Client - there are lots of these, this seems like the most usable to me

React Developer Tools

Resolution Test - Easily switch resolutions for responsive design testing

WebGL Inspector - My favourite. The killer app of WebGL debugging. If you do anything with WebGL this is a MUST, for tracking down memory leaks to debugging state changes to viewing GL calls...