There are several websites out there that let teachers quickly assess student comprehension. Most allow the students to answer questions using any device (smartphone, iPad, computer, etc) and can show graphs of results. Question types often include polls, multiple choice, true/false, and other non-divergent responses. Some of the websites may allow for saving the questions so that you could create a more summative type assessment for students to take.
These websites are great for pinpointing student understanding and allowing the teacher and students to progress at an appropriate rate during a lesson. They replace the "clickers" of interactive whiteboards and update the tried-and-true practice of exit slips, tiny pop quizzes, or other fast assessment tools. It could even replace "Please raise your hand to show you understand" and the chaos that can ensue in a large classroom. Because of the results data, teachers can find common misconceptions and more easily target them.
Listed below are some of the tools I have come across. This is by no means a comprehensive list, nor do I favor one over others (in truth, I haven't had an opportunity to try them all.) Do any of these pop out at you as especially worthwhile?