For frontend dev, I use:
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...
