As I wrote on my Medium account, I would suggest to limit the amount of extensions you add to your browser, since they tend to eat up a lot of RAM, which slows down your computer. If you're using Google Chrome, these processes appear as "Google chrome helper" in the activity monitor. But there's an extension I would not remove from my browser at any cost. It'a a colour picker called Colorzilla.
It enables you to basically pick the colour of any element in your browser. It's pretty handy if you're working on a website for a client who didn't give you any identity guidelines. You simply click on the element bearing the colour you're looking for, get the hex code, copy it to your clipboard and store in an easy accessible notepad, e.g. Evernote Quick Note, for fast access if you need to use the same colour repeatedly in your workflow.
See for instance how I picked the colour of my own logo on Callmefred.com using Colorzilla.
The hex code for this colour is #e77d74. BTW, it's also archived in the picked colour history section of the extension.
I can now use this hex code for instance on The Noun Project, which recently added the possibility to change the colour of the icons just before downloading them.