Questions to speaker per Smartphone in University lectures
This free guide - the Event App Bible - lists lots of conference/event apps. I believe there are a mixture of free and paid-for systems included. You should look at the "Audience Response Systems" to id apps that offer this type of functionality, and cross reference that against the pricing table. Hopefully you can find something that works for you. (The guide comes from the Event Manager Blog and was updated recently (2015))