Today Google launched add-ons—new tools created by developer partners that give you even more features in your documents and spreadsheets. To browse through add-ons for Docs and Sheets, select Get add-ons in the Add-ons menu of any open document or spreadsheet. Once you install an add-on it will become available across all of your documents or spreadsheets and you can start using it right away. Note that add-ons for spreadsheets are only available in the new Google Sheets.
(How to turn on New Google Sheets? Turn on the new Sheets by checking the “Try the new Google Sheets” box in Google Drive settings. From then on, all new spreadsheets you create will work offline and include these new features. We’ll be adding a small list of missing features in the coming months, so if you rely on any of them, you may want to wait a little longer before opting in.)