I used Poll Everywhere which allow teacher to take polls of student answers. This would be a great warm up at the beginning of the class. I could see this as an advantage when covering difficult concepts.Teach and reteach could be done more quickly.
The second was Wallwisher. If a student was absent and could not get instruction you could video one of your lectures and place it on your wall. They could then place questions on the wall and you could give them answers back. This could be used to find out what the class likes in you presentation skill and what they might not like.