This is a Mac product, they also have a Windows based app in beta. If you want to try out the Windows based app wait until March 25 when they will release a new beta. This is not free, but it looks interesting as a course design tool.
Socrative is a smart student response system that empowers teachers to engage their classrooms through a series of educational games and exercises via smartphones and tablets. Our apps are super simple and take seconds to load and run. Teachers control the questions and games on their laptop, while students respond and interact through their smartphones/laptops.
I've written about alternatives to proprietary clicker systems in the past. Usually when I mention those alternatives, I mention using text messaging. However, in the last three weeks I've presented in three different countries. In two of those countries I wasn't able to text (at least not without incurring ridiculous charges) so the only tool for polling students that I showed was Socrative.