Plickers is a relatively new "low tech" student response system. The student "clicker" is a paper print out of an abstract image (similar to the concept of a QR code) the students orient to provide their preferred answer.
To gather responses the teacher uses the Plickers app (iOS or Android). Each unique code can be printed out, so that each student has a "clicker" for responding. The teacher then scans the cards to record the student answers.