Socrative can be used for live quizzing or online quizzing, with students answering in their own time.
Teachers can register for a free account and use it to save and activate your quizzes. In the free version, only ONE quiz can be activated at one time, though you may save many different quizzes in the quiz archive.
Website: http://www.socrative.com/
Here is the comprehensive User Guide for Socrative:
Click on the link below to download the PDF.
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Open in new tab Here is an instructional video:
How to use Socrative and Powerpoint in tandem: