Just came across this web site, and think it is something everyone might want to check out. It basically is a student response system that a teacher controls. It can be run from any web browser, so you can use a desktop computer, a laptop, iPhone, Android tablet, etc. This is a very flexible system. It is currently still in Beta testing, but you can sign up for free. Once they go live production, it sounds like they will start charging, but those that sign up now, won't have to pay.
If you are looking for a 'clicker' type system to interact with your students, then this is definitely worth checking out. Students can use their Smart Phones or laptops to become involved. You have to be willing to let your students use their phones in class, and while this might not be right for everyone, it is definitely something that could benefit a classroom if incorporated properly.
Below is a video they have on their web site....so go check it out at Socrative.com
"Socrative is a smart student response system that empowers teachers to engage their classrooms through a series of educational exercises and games via smartphones, laptops, and tablets."