Plickers is my favorite student response tool for classrooms in which not every student has his or her own tablet or laptop. Plickers makes it easy to semi-anonymously gather feedback from students. Students simply hold up a card with a QR code to vote and you scan the cards with your phone or tablet. You can scan the whole room in one swoop and have results instantly appear on your screen.
I've used the Plickers iOS app on my iPad for years now even though it wasn't optimized for iPad. Last week Plickers updated to be optimized for iPad. This is not a major update, but it is convenient for teachers who use Plickers on their iPads.