Add Reminders lets you simply enter reminder messages and email addresses, then specify a date you want your reminders sent. The Add Reminders Add-on allows you to send the same reminder to everyone in or you can send individualized reminders.
Flippity provides a Google Sheets templates for creating quiz games, audio flashcards, bingo cards, and progress trackers. Flippity's Google Sheets Add-on makes the template available with a couple of clicks in any of your Google Sheets.
Flubaroo is a popular Google Sheets Add-on that grades student responses to a quiz created in Google Forms. The autograde option in Flubaroo allows you to have students automatically receive their scores after submitting their responses.
Online Rubric is a Google Spreadsheets Add-on that enables to you create rubrics, enter scores, and email scores to students all from one place. Online Rubric provides clear instructions for each step of the processes of creating a roster sheet, creating a rubric, and emailing grades to students.