The chrome browser is an invaluable tool as a developer through the included DevTools. It is ideal for troubleshooting because it allows you to see how the page is being rendered behind the scenes. Ctrl+Shift+i (Windows) or Cmd+alt+i (OSX) will become your best friend.
uBlock Origin is a must have for any internet user. It blocks scripts and ads while you browse the web. This is helpful for maintaining your privacy, prevent you from inadvertently clicking many of the malicious links out there, and generally making your experience better by blocking ads. I do disable uBlock for sites that I trust and support as many rely on ad revenue to survive.
EyeDropper is a great tool for lifting colors from a website. Very helpful when you see a color you love and want to get the hex value so that you can use it on your site.
Edit This Cookie lets you view, change, or delete cookies in your session. You can achieve this through Google Chrome’s DevTools as well but I find the extension to be more accessible.
HTTP Spy lets you inspect headers.
Full Page Screen Capture helps you gets screen shots of a website. This is particularly helpful when you want to capture parts of a site that would require scrolling to view.
Super Auto Refresh lets you automatically refresh a page at an interval of your choosing.
Browse the chrome extension store to find more great extensions! Keep an eye on the ratings and reviews because there are many extensions that are no longer supported or which do no behave as advertised.