"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," informs Google. You can always find them in the Add-ons menu.
There are add-ons that translate the selected text, find synonyms, create a table of contents, insert maps, create charts from spreadsheets, generate bibliography, check your writing for consistency, add mail merge to Google Docs, print address labels, remove duplicates, import Google Analytics data and more.
Here's the Translate add-on in action. Select some text, go to the Add-ons menu, click Translate and Start. You can select the destination language in the sidebar that shows up. For some reason, only a few destination languages are supported.