I discovered Plickers after an observation of a classroom last year. I have used clickers in my classroom, but have issues with batteries and can only retrieve reports after we are done reviewing.
Plickers uses a picture that can be scanned by a smart device. Depending on how the student orients their paper, they can have up to four different choices. Plickers is a free resource that you can create classes, question banks, and reports that show student percentages as well as question percentages. I use it for review. It offers real time data so you can see how your students are doing as you pose the question.
I am all about free resources!