Clickers allow teachers to poll attitudes, check comprehension, stimulate discussion, and more. And everyone loves a toy!
Teaching with a Classroom Response System (overview from Vanderbilt)
Multiple Choice Questions You Wouldn’t Put on a Test: Promoting Deep Learning Using Clickers (beyond questions with a "right" answer, from Toward the Best in the Academy, a publication of the Professional & Organizational Development Network)
Classroom Response Systems: A Teaching with Technology White Paper (Carnegie Mellon)
How to Use Clickers Effectively (University of Colorado Science Education Initiative provides this video)
Clicker Bibliography (more than 280 entries on every conceivable topic from Derek Bruff, Vanderbilt)
Socrative (student response system that works with any internet-connected device, allowing multiple question types, quizzes, exit tickets, and more. No clicker necessary because students use their own devices)
Poll Everywhere (classroom polls with images, equations, and more using student smart phones or laptops)
Net Click lets you convert existing slides into a presentation that allows student interaction and polling.