Add-ons for Google Documents and Sheets. Those features were initially added to consumer Google Accounts (@gmail accounts). Yesterday, Add-ons were added to Google Apps for Education domains. If you are a GAFE user and have not seen Add-ons appear as an option in your Google Drive account, contact your domain's administrator. GAFE domain administrators can enable or disable Add-ons for users.
Applications for Education
Some of the Add-ons for Docs and Sheets that jump out as being particularly useful to teachers and students are EasyBib for creating bibliographies, Kaizena shortcut for adding voice comments to Google Documents, and Table of Contents.