This guide is for those who are using a Chromebook without having installed another operating system (Chrubuntu or Ubuntu via Crouton); that is, you're using Chrome OS. These extensions are pretty useful all the time, though, so check them out!
When you're using a web browser as your only application, your needs change and your consumption of the internet changes as a result of the OS you're using. For instance, if you're trying to write something in a traditional OS, you go to your word processing application and write it. If you're on Chrome OS you're going to have a tab for your word processor amid a sea of other tabs vying for your attention. You can cut down on distractions with StayFocusd, which you can configure to give a limited amount of time to distracting websites (naturally, I have mine set up to only allow a few minutes each day on Facebook, but anything you habitually flip over to can be put on the list. They even have an option intended for Reddit users which makes it so that when you click through a link on your blacklisted site, browsing the new site counts against your time). It's really been helpful for me as I've brought my daily use of Facebook from probably 45 minutes a day down to less than 5.
Your OS probably allows for quite a few keyboard shortcuts, and so does Chrome OS. Some of the web applications you'll be using have their own keybaord shortcuts, like Google Docs. But enabling some really powerful keyboard shortcuts across the entire browser requires using an OS-specific tool like AutoHotKey or Autokey or Automator. How about bringing some handy new ways to manage your browser to the keyboard with Vimium. Particularly if you're using one of the older CR-48s whose trackpads are supposed to be sub-par, you'll be thankful for the way you can almost entirely give up using a pointer. As a fan of tiling window managers myself, Vimium is crucial to my being able to work efficiently on Chrome OS.
This one is the raison d'etre of creating this list in the first place. Chrome OS should simply not be run without using this extension. Here's why: Chromebooks are intentionally a bit lacking in the hardware department because the software is so light. That's why we get the amazing price. But because the hardware (specifically, I'm talking about RAM) is lacking, Google had to build some rather overzealous memory management into Chrome OS, so you'll find that if you have more than 5 tabs open, tabs will be suspended when they haven't been used for a bit and then reloaded once you go back to them. For the most part this doesn't cause any problems because Chrome OS can remember your place and it tends to maintain your text input into forms, but in the case of any "infinite scroll" web applications like Twitter, the reload will bring you to the very "top" of the page, which can interrupt you in an exceedingly annoying way.
OneTab lets you limit this by having one tab that backs up the tabs you aren't currently using. Rather intuitively, it puts all your Tabs into One. This cuts down significantly on RAM use and makes it so that you can click away from your Twitter tab without it getting reloaded and losing your place. It also prevents the small time delay it takes to reload your suspended tabs AND keeps your performance speedy because your memory isn't tied up on tabs you aren't currently viewing. It essentially puts you in control of memory management rather than letting Chrome dictate how it's done. If you take anything away from this list, take OneTab. I wouldn't run a Chromebook without it.