Quick responses with immediate feedback.
Community Review for Poll Everywhere
My Take
Poll Everywhere is effective because it's simple. It's basically a whiteboard your students can write on from their cell phones, and their responses show up close to immediately, so you can see them and give immediate feedback.
How I Use It
I've used it to take polls and build word clouds based on student responses to questions about how our PBIS values should be demonstrated in the classroom, then used those word clouds to build class rules.
I've used it to name items of different cultures and American culture to get students thinking about the metaphors of melting opt and salad bowl.
I've used it as a warm up for students to put up responses to activate background knowledge before reading something new.
I probably haven't scratched the surface of good possible uses. It hasn't gotten stale yet.