Mentimeter is a free service that allows you to pose a question to your audience and get instant feedback on that question through cell phones, tablets, and any other Internet-connected device. I reviewed the service back in February of this year. Since then Mentimeter has add a new open-ended response format.
Mentimeter allows you to create an unlimited amount of questions and collect unlimited responses. The user interface is clean and simple and your students don't need to create accounts in order to respond to your questions.
A demo of Mentimeter's basics is included in the video below.
Applications for Education
Mentimeter, like Socrative and Poll Everywhere, is a good tool for collecting informal feedback from your students. You could use Mentimeter to ask students simple questions like, "do you feel ready for the quiz on Friday?" then use that information to formulate your next lesson plan. Tools like Mentimeter are also good to use as exit ticket systems at the end of a class meeting. Again, you can use the information collected through those exit tickets to influence how you design your next day's lesson plan.