EverySlide is a free ( for educators and students ) service that allows you to share your slides directly to the iPads, laptops, Chromebooks, and Android devices used by members of your audience. As you move through your shared slides you can pop-up a poll to gather feedback from your audience.
EverySlide supports PowerPoint and Keynote slides. To get started just upload your slides to EverySlide (you can re-use uploaded slides for multiple groups). When you're ready to present give your audience the access code for your slides. At the end of your presentation you can grab a spreadsheet of the responses to your poll questions. Watch the video below for an overview of EverySlide.