Ask HN: Best Google Docs add-ons?

What are your favourites? Form Mule has been brilliant when combined with Google Forms. Create a mail merge from a Sheet and then send each recipetent a customised Google Form link. When they complete the form the data is inserted back into the mail merge sheet. https://chrome.google.com/webstore/detail/formmule-email-merge-util/kabhamaiihmaphgpijdolgcihofefajn?hl=en