Ask HN: What are the best browser extensions for beginners learning to code?

I'm working on a blog post for our Thinkful students that'll include valuable tools and browser plugins for a beginner. Examples: Extensions What Font Page Rule Perfect Pixel Eyedropper Sites/Apps Hipster ipsum Place Kitten Hovt Fake image Intermediate/Advanced Stylebot Cookie Inspector JSONView What other tools/extensions have you found valuable? As you can see, I'm not looking for text editors or frameworks but fun little hacks/tools :) The final post will include a line or two explaining the usecases each app. p.s. do the categories make sense? I was originally thinking of sorting by use case (image editing, copy related, etc).