Writing a Chrome extension is incredibly easy! You don't need any special tools either, just a text editor and Chrome. Create a folder and make a manifest.json and an index.html file. Fill them in with the information below and you are ready to go!
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.