Socrative is a free digital tool that allows you to interact with your students in a variety of ways. It can be used to test students, to poll an audience, the carry on back-channel conversations as well as an effective way for students to provide exit cards after a class. The app is completely free and works on any device with an internet connection.
As we move towards B.Y.O.D (Bring Your Own Device), tools of this type will become even more valuable. The potential for organizing data and promoting a paperless environment is intriguing. I have played around with Socrative and I have found it really intuitive and easy to use. You can find a full review on Edshelf.