Kahoot, currently in beta testing, is a "game-based student response system." It works by letting teachers create multiple choice quizzes online, and then providing an interface for students to compete on the quizzes. To get started, teachers can register for a free account, and then make their first quiz, called a "Kahoot."
Questions are multiple choice, and provide a symbol/shape and color for each answer, which also displays on the student screen. Teachers can control the number of choices, as well as the time limit. The quicker a student answers correctly, the more points he/she will receive. After each question, a "leaderboard" appears, so students can see how well they did.