March 11, 2014: Google has launched "Add-Ons" for both Docs and Sheets. This allows users to access new tools created by Google developer partners. The tools give users more features in documents and spreadsheets. You can see which add-ons are available when you are working on any doc or sheet. A "Get Add-Ons" item can be seen in the "Add-Ons" menu at the top. Click to browse available add ons, and install them. Once installed, the add-ons will be available in all your documents and spreadsheets. Add-ons for sheets are ONLY available in the new version of Google Sheets.
Please note: In Google Apps domains, the administrator must enable add-ons through the Drive settings in the Admin Console.