Best way to poll students during a class by web
Poll Everywhere is a good, easy one. It's free for 25 responses per question. Students can respond on their smartphones, laptops, or even old-school cell phones via texting.

Plickers is another good one. For this one you print out the possible responses and hand them out. Then students hold up the card corresponding to their answer. You use your smart phone camera to scan the room and the app calculates the responses.