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.

An example of what you are looking for is a system called ARSnova. It is a system which allows the students to answer questions or give feedback during a lecture on their smart phones. The system allows to generate a session code (which is a six digits number) which the students have to enter in oder to join the session for the lecture. Alternatively, a QR code can be shown which directs the stundents directly to the session.

As far as I know ARSnova is Open Source and is developed by the Technische Hochschule Mittelhessen.

The best implementation of such a system that I am aware of is Learning Catalytics, which was developed by Eric Mazur (and team) at Harvard and is now owned by Pearson.

https://atg.fas.harvard.edu/learning-catalytics

I used it personally at a demo talk - it is easy to use for both instructors and students, is accessible on any device, displays analytics of the student responses, and also facilitates interactions among students after the responses (e.g if you have a multiple choice question, once students have responded, you can use it to group up nearby students that have different answers so that they can try to convince one another).