Two of the best I have found for classroom use are discussed here:
SocrativeSocrative started out as an independent testing tool and was quickly snatched up by MasteryConnect. It provides a free, simple to use interface that allows teachers to create and deploy their own tests. Students can connect to assessments using any web enabled device (class computer, iPad, Chromebook, phone, etc...).
When you create your account, Socrative provides a Room number that students connect to. From there, you can deploy pre-created quizzes/tests, or as a quick question on the fly. Best of all, with Quick Questions, you can also allow students to vote on the best answer to create discussion.
This site is great for stepping up student engagement. If you've ever been out to a restaurant and you played trivia on the TV monitors supplied by the chain, then you'll get the idea of this tool. We recently started using this during lunches to draw students into the cafeteria.
Kahoot allows you to create your own question sets, or choose from over 1 Million public question sets over a wide variety of topics. You can find something for everyone, ranging from Trivia Crack to Math and Science.
Students can use cell phones or computers to engage with content. A Game PIN is provided when you launch a question set and students join in by entering the PIN and a nickname. A leaderboard shows the top 5 point earners to create a competitive spirit. Great for reviewing for tests!