This is my favourite classroom tool at the moment. Create and play quizzes, surveys and discussions using any device with a web browser. You can make your own, use the public kahoots created by other teachers or find out what your students really know by getting them to create their own Kahoot.
Here is a presentation I made for a TeachMeet session.
Here is a link to my Kahoots that I have created:
Click here to play a kahoot. (You will need a game pin to play)
Watch this video about Kahoot created by Susan Johnson.
Or this one created by Tim Smith.