Yes, I am going to become a "listicle" terror. Blame Thane.
As a user of (almost exclusively) Chromebooks and Chromeboxes since 2012, I've used quite a few extensions. Some I've discarded or at least disabled. Others make my life better and/or easier and I'd hate not to have them. Here (in no particular order) are five of the latter kind.
The Great Suspender -- This extension "suspends" tabs that haven't been visited for a certain (user-set) amount of time to cut back on memory usage. You can whitelist sites that you never want suspended, and when you go back to a suspended tab, you can click on a link and reload the web site that was running in that tab. The Great Suspender was my first solution to the slowdown phenomenon that comes with running lots of browser tabs on a machine with only 2Gb of RAM (my Asus M004U Chromebox, which I upgraded to 4Gb last week), and it helped. A lot.
uBlock Origin -- Yes, I finally went over to the dark side and started using an ad blocker, for the same reason I use The Great Suspender. That is, to not have my RAM eaten up by a bunch of extraneous crap. Yes, I still feel guilty sometimes. No, not really. uBlock Origin was the extension I settled on after trying several, and I'm quite satisfied with it.
Keep Awake -- The Chromebox doesn't come with user-adjustable power management settings. You can get around that by going to command line in Chrome Shell ("crosh") and directly editing crap. But then you'd have to remember what you edited and how if you wanted to change or undo it. Keep Awake gives you three convenient settings to switch between with a click. Normal (after a certain amount of time, the machine goes into sleep mode and the displays go dark), a setting that keeps the system awake (and doing stuff if you have something running) even when the displays shut down, and a setting that doesn't even let the displays shut down. When I have the Chromebox web mining cryptocurrency overnight while I sleep, I use that middle setting.
Screen Shader -- A Chrome extension take on f.lux, a program that adjusts the color tone of your computer screen to "to decrease eye-strain, eye fatigue and to appease your brain's day/night cycle." I've only recently started using it, but it feels easier on the eyes. I suppose I might come back later and deprecate it. But I doubt it.
File System for Dropbox -- On most operating systems, if you want to use Dropbox you install some software that creates a Dropbox folder where you save the stuff you want synchronized/backed up to cloud storage. ChromeOS isn't most operating systems. Until this extension came along, if I wanted to store a file in Dropbox I had to go to the web site, log in, and upload the file. File System for Dropbox appears in my file manager as an additional drive, and acts like one. I can save to it, load from it, copy files to and from it, etc.