Socrative: Responsive Web-Based Audience Response
To start, log in to your instructor account, set up a "room", establish a class session, and create a set of questions. During class, provide students with the room number, which they can access (without an account) via any web-enabled device.
As an instructor you can mete out the questions as the class session progresses or pose ad hoc questions. Student feedback is returned instantaneously to your web browser (or smartphone or tablet) where it can be shared with students. At the end of the class session, the system will send you a report containing the questions and answers. Socrative also offers flexible "open-ended" questions that can be difficult or impossible to deliver via other clicker solutions. Socrative does not register students so it cannot reliably be used to administer graded quizzes or take attendance.
If you're amenable to students using their devices or laptops in the classroom (some instructors aren't) or just want to use audience response sporadically during the semester, this is an excellent alternative to dedicated clicker devices.