Simply Send is a Google Forms add-on. It allows you to simply send the form submission results to a list of email addresses as well as to an email address available in the form submission data.
Choice Eliminator 2 is a Google Forms add-on. Choice Eliminator will eliminate options from a multiple-choice, dropdown, or checkbox type of question. Great for signing up for time slots or having students choose topics without doubling up.
Sheet Navigator is a Google Spreadsheets add-on. It allows you create table of contents for sheet, by opening a sidebar listing all the sheets. This allows easier navigation between sheets when there are too many spreadsheets.