Google have just rolled out a new feature for Docs and Sheets (only the new-style sheets), namely their Add-Ons tool.
Inside a Doc or Sheet you’ll now see a new item, Add-Ons, toward the right ofthe toolbar.
This opens a new dialog where you can review and select the available add-ons.
There’s a broad range of them, including tools to aid printing, translation, graphics, fax, workflow and much more.
Once you install an add-on via this service, it’ll be available across all of your documents and (new-style) spreadsheets.
Why not load it up and explore?