Geddit is a free, web based student response system, available on any device, that allows teachers to quickly gauge student understanding. Students use the app and indicate their level of understanding on a 1-5 scale. This formative assessment lets teachers know if they can move on or not. Feedback is private and real time. You can also use multiple choice and short answer assessments.
http://educationaltechnologyguy.blogspot.fr/2014/08/geddit-web-based-student-response-system.html