Kahoot is another great Rapid Response source. It's free and easy. I'm going to use it at the Summer RILINK Conference to survey the group I'm presenting to, but it can also be used for Quizzing students, or classroom Discussions. It's free and the kids don't need email addresses. They just go to kahoot.it and put in the pin number you display at the front of the class. They can access the kahoot with any type of device, including phones, as long as they can get to the Internet. Fun game show music and a timer make it exciting for the students.
Here's a tutorial that helped me get started.