You can find Google Forms Add-ons by opening the new Add-ons menu that appears at the top of your Form editor. After opening that menu select "get add-ons" to browse the gallery of Forms Add-ons.
Applications for Education
The Form Limiter Add-on mentioned above is useful for delivering timed assessments. Form Limiter can also be used to close the form when you a designated number of submissions have been made. That option is useful when you're using Google Forms to create capped registration lists.
gMath for Google Forms is another that teachers will find useful. gMath allows you create and insert graphs and mathematical expressions into your Google Forms. That feature is one that math teachers have wanted for years.