goFormative.com has been my favourite tool to use in the classroom this year. It enables me to give every student a voice, see misconceptions and thought processes in real time, and feedback and guide students the moment they need it.
Anyone can set up a free account and there is account integration with Google which is great. You can also easily add assignments to Google Classroom for quick student access. There is such a variety of question types, and there is an option to upload and transform pdf documents to help you go paperless!
I recently gave a workshop at the AppsEvents Sweden Summit to a group of teachers, you can get those slides here.
This video shows how to use the tool in under 5 minutes:
If you wanted more information you can find it here:
I know that there are lots of free student response systems out there, but this one seems to do everything I could want. I'd strongly recommend trying it out!