Kahoot is a gamified way to do fun formative assessments. Teachers advance through questions at the pace they control, and students earn points for how quick and accurate they answer. Leaderboards after every question let students know their standings. Teachers can run reports afterwards to determine individual answers and other useful data such as response time. One of the few possible drawbacks is that it requires a central display (such as a projector) for students to see the questions and possible answers. Read this blog entry for more information.