This is a list of software and tweaks from someone who only uses a chromebook
1) Not software, a tip - Creating Web Apps
Ok, so this isn't creating a PWA or anything anywhere near as advanced as that. It will create a link to a webpage which will inturn open that webpage as a Windowed App.
In Chrome click on the Hamburger, then click on More Tools follwed by Create Shortcut
This will open the Create Shortcut Window and the important thing here is to make sure Open as Window is clicked
Click on Create
As an example of this I use Axigen as an internal only webserver for all my systems to mail stuff to and I have this running in Windows Mode as an app.
2) Bookmark Sidebar (Free and Paid)
If you have a lot of bookmarks in your bookmarks bar and want to reclaim that space. this is a nice sidebar alternative.
3) Boomerang for Gmail (Free)
I love Boomerang, i can schedule emails to go out at set times, i can check it arrived and set reminders if i have not heard back.
Boomerang allows you to schedule messages to be sent or returned at a later date. Write a message now, send it whenever, even if you're not online. Track messages to make sure you hear back, and schedule reminders right inside Gmail™. Know whether your email got read with cross-platform read receipts. Free yourself from constant interruptions using Inbox Pause, while still getting notified about the emails that matter.
4) Chrome Remote Desktop (Free)
Having gone through various remote desktop solutions, i always ignored Chrome Remote desktop. Now its the only one I use. I only have 1 desktop I want to connect to my Mac from a remote location. Chrome Remote desktop is fast, works over the internet (ans is still fast over 3G/4G) and seems to scale accordingly.
5) Keep Awake (Free)
Sometimes when I lock my screen I want my screen to stay on, sometimes I want the screen to lock and go dark, other times I will leave it to the default settings.
Keepawake is an extention where I can choose if my Chromebook is going to stay awake or go to sleep.
6) Wicked Good Unarchiver (Free) - ChromeOS Only
Open 7z/ar/CAB/CPIO/deb/ISO/jar/lha/pax/rpm/tar/warc files in Files app.
I deal with a lot of Tar and RPM files and this was the extention which provided me with the ability to open these files in the File Manager.
7) CodePad (Free/Donate)
I use Codepad the same way Notepad/Wordpad were used on Windows. Its my goto text editor. This is doing CodePad a HUGE disservice as the lin above shows this is a full featured code editor, well thought out and very quick to use.
8) Dochub (Free)
Securely edit, fax and sign PDF and Word documents for free. Fill forms. Merge documents and reorder pages. Create templates.Sign, edit and share PDF and Word documents online
Dochub is a full environment to manage PDF and Word Documents, at its very base its a replacement for DocuSign. I've been using it for editing PDF's and Signing PDF's.
9) Office Online Apps (Free)
If you don't want to use Google Docs to edit your document (and you should) Microsoft have provided a plethora of plugings to assist editing documents of all types in Office using the online Web interface.
10) Theatre Mode for Youtube (Free)
Theater mode opens the Youtube videos so the vide takes more space on your screen. Even if you set it in YouTube when you restart the site YouTube will revert to the default view. This extention ensures each time you open youtube it opens in the wider Theater View.