Today we're launching 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.

Add-ons are enabled by default. Google Apps admins have the option to disable add-ons through Drive settings in the Admin console. Users will always see the Add-ons menu and can browse them but will not be able to install them if the feature is disabled by the admin.

Scheduled release domain admins have the option to disable the feature in the Admin console before it launches to their users in a few weeks.

Release Track:
Add-ons menu: Rapid release
Admin console setting: Rapid release and Scheduled release

Editions included: 
Google Apps for Business, Education and Government

For more information:
https://support.google.com/a/answer/4530135
http://googleenterprise.blogspot.com/2014/03/bring-little-something-extra-to-docs.html
whatsnew.googleapps.com
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