Why is PollEverywhere my hands-down personal favorite polling tool? Because the students can text in their responses from their cellphones. No more response systems, with their added hardware and software, necessary for the classroom! Very millennial . . .
You can use PollEverywhere for free, but there are certain limitations. With the basic account:
How to Create a Poll in PollEverywhere
How to Customize a PollEverywhere poll