Add-ons in Google Docs are tools built by third-party developers using Google Apps Script. You or your students can install add-ons to bring more functionality to Docs, Forms, and the new version of Sheets.

To access Add-ons first open a new Doc, Sheet, or Form then click Add-ons in the menu bar and Get Add-ons

Here are a few:

 Doctopus – This is a document management script to use for student projects. It allows you to auto-generate, pre-share, and manage grading and feedback on group and individual projects. Start with a class roster in Sheets.

 Flubaroo – This script allows you to automatically grade multiple-choice or fill-in-the-blank assignments using Forms. It also computes average assignment scores, average score per question, and highlights low scoring questions.

 EasyBib - The easy bibliography and citation generator integrated into Google Docs

 Kaizena Mini - Highlight and speak your comments in Docs instead of typing. Track skills so students know what to improve. Avoid repeating yourself with quick links to any online resource. Kaizena Mini brings the full power of Kaizena feedback right into the Google Docs Editor and is 100% compatible with Google Classroom.

 (g)Math - Create graphs and complex math directly from the sidebar into a cell in your Google Sheet. 
You can use commands or the prebuilt codes to create complex math that is not possible using the built-in Equation Editor. Some examples include Geometric signs, custom characters, and formulas (like the Quadratic formula). Also available in Docs.

 OpenClipArt Now you can search and choose from more than 50,000 clipart images provided by Openclipart

 LucidChart Diagrams - Quickly create and insert flowchart, UML, wireframe, mind map, and more diagrams directly into a Google document.