I’ve been using Socrative in my classes more and more lately. It is an online polling tool that lets me post a question or problem, and my students can anonymously respond. It’s very flexible – you can offer multiple choice, true/false, short answer, etc. Here’s a screenshot:
They love using it, and I get immediate feedback on how many students understand what we’re learning, and how many need some extra practice. It works with smartphones as well as laptops.