I've been using a Personal Response System in my classes. Students have a remote control in which they respond to multiple choice questions I periodically display during a lecture. Students vote anonymously and then I show the answer. It really seems to help students realize what facts were important, and a chance to practice application of what they just learned. We chose a system sold by Turning Point Technologies and it has worked well. They advertise that you can create slides on the fly, but I do not recommend it. I suggest planning questions that illustrate the most important concepts from your class. Students will then not be able to say they did not know what you wanted them to know.