I got interested in Mentimeter because someone used it as an icebreaker in a course I was on. It is free and has lots of possible uses. You can also use a pay version but I don’t think that is really necessary. I used it in an experimental class with a content lecturer; I was there to support the English language side of things.
Students could vote anonymously on questions/exercises which the lecturer put to them: this is good because the teacher can see what has been understood/perceived and adapt as necessary.
Another useful idea was to invite the students to ask questions which were projected for everyone to see and the lecturer to feedback on near the end of the lesson. Frequent questions were grouped together and so it was easy to prioritise which to answer. It was also apparent to the lecturer that this way she was getting a deeper insight into what students know, what they don’t understand and want to know more about. She reported that this does not happen to the same extent without a tool which allows anonymity.