There are many 'Top X useful extensions for web developer' posts out there. The problem I have with them is that they recommend multiple modules that do the same thing. For example, the 'web developer' and 'pendule' modules both offer a color picker tool, so I don't really see the need for another special extension just for color picking. Same with browser resizing, why would I want 3 extensions installed for that? Also, everyone should already have a method of taking screenshots (full or partial.) Even Windows comes with a basic snipping tool that does a perfect job. I prefer not to have a huge bloated list of extensions on my browser. I've boilted it down to just 4 addons. I consider all these must haves.
Web Developer - This is like the mother of all web dev plugins. It does the job of dozens of other plugins. Tons of options and tools to use so it takes a while to get familiar, but it is very powerful.
- Cookies - View, edit, delete cookies
- CSS - View styles, disable all styles
- Forms - Clear forms, show password fields
- Images - Disable images, find broken images
- Page Info - View used colors on a page or any other info
- Tools - rulers, outlines, sizes, eyedrop color picker, etc
- Resize window - good for testing various resolutions
Lorem Ipsum Generator - Generate latin dummy text for place holders
Color Generator - While the Web Developer extension has an eyedropper color picker, it doesn't have a color generator to allow you to generate your own colors. This simple extension allows you to create web colors.
ScriptNo - Disables all javascript by default. Temporarily allow or permantently trust domains. This is actually not too useful for a web developer, but it is important for security. I recommend this extension for anyone and so I have included it here.
EDIT:
Here are some more extensions I have come across that I found noteworthy:
https://chrome.google.com/webstore/detail/utime/kpcibgnngaaabebmcabmkocdokepdaki/ - Utime - For converting UNIX timestamps to something readable
https://chrome.google.com/webstore/detail/json-prettifier/kccpfgilgmgbipamhohknpokhibinhhj - JSON Prettifier - for viewing JSON objects