Socrative
Socrative is a great formative assessment tool. Socrative empowers you to engage and assess your students as learning happens. Through the use of real-time questioning, result aggregation, and visualization, you have instant insight into levels of understanding so you can use class time to better collaborate and grow as a community of learners. You have the ability to create quizzes, polls, and exit tickets. You can also use quick questions to assess students on the fly.
The secretive website has a great overview video that explains all of the features.