- A relatively new one from Appirio: What Are the Best Add-Ons for Google Docs and Sheets?
- InfoWorld: 8 essential add-ons for Google Docs and Sheets
- InfoWorld: 8 free apps that pump up Google Drive
- labnol.org: The Best Add-ons for Google Docs and Sheets
- BetterCloud: 12 Free Add-Ons That Take Docs and Sheets to the Next Level
- coolcatteacher.com: 15 Best Google Drive Add-ons for Education Add-ons for Education
One thing I've noticed about these lists is that Lucidchart Diagrams appears many times. I'm going to have to try that one.
And, of course, the list wouldn't be complete without the official Google Apps for Work Marketplace and the Google Add-ons Developer page.