With NetClick, you can make your existing slide content interactive.
Upload your presentation. Your slides will be synchronized on any connected students' mobile displays. Get instant, visual feedback on what part of a slide is confusing. Students can indicate exactly what part of diagrams, pictures or text they want to discuss — in an instant!
Via Baiba Svenca
NetClick enables you to make your presentations interactive which is perfect for getting student feedback instantly. This is extremely useful when you are explaining new things or demonstrating objects or textual content.
The service is free for teachers if students pay $5 per semester but there are some other licence options.
Looks like this might be worth checking out. Solid research beind the use and results.