Teacher Login and Setup: https://getkahoot.com/
Student Page: https://kahoot.it/#/
This is the hot app that I see being used in a lot of classrooms ranging from Kindergarten to College Classes. This was even used in my Graduate classes. A great method for adding gaming elements to your formative assessment in your classrooms (or next professional development session). Even fun to play other quizzes that other teachers make.
Works on any device with an internet connection and web browser; Tablets, Chromebooks, and Computers.