If you haven't taken a look at Google Add-ons recently, a lot has been...well, "added on." Add-ons are tools built by third party developers using Google Apps Script to provide additional functionality to Docs and Sheets. Helpful Add-ons include ones for mail-merge, digital signatures, customized spreadsheet formulas, and much more. You can install Add-ons right from Docs or Sheets and then choose to enable the add-on in only that doc or sheet, all docs and sheets, or none. For more information see Overview of add-ons.
*Please keep in mind that extensions, add-ons, and labs are not supported or covered by U-M's Core Services agreement with Google.