Popular, perfect for ConcepTests, but functionally cumbersome.
Community Review for Plickers
I like the way this is used in the actual class, but it is very difficult to set up. There is no import of questions from any other source, so previous resources, and all the time spent on creating them, becomes a waste. Most annoyingly, you cannot select more than one question to put in a queue. You have to go to each individual question, select it, then select the class queue to put it in, and you can't even select more than on CLASS at a time. Technical support is quick to answer your questions/problems/suggestions, but they are not quick to integrate suggestions. I have been using this for a few years and I keep mentioning the above issues to no avail. They did create "sets" of questions, but that does not address these issues and they still have to be created anew. This could be a superb tool, but it is not at present.
How I Use It
I like Eric Mazur's ConcepTest model, and this is an excellent resource for that. The Plickers cards are a brilliant way for students to vote anonymously and independently since they cannot see how anyone else voted.