Google's just added third-party plugins to Google Docs and Sheets. Called Add-Ons, they enable tasks like printing address labels or getting approvals on documents—similar to the apps Microsoft Office users are familiar with.
Once you install an application, it becomes useable straight away across all of your documents and spreadsheets. This does raise a question of whether allowing third-party applications access to potentially-sensitive documentation is completely safe. [Google via PCWorld]