Probably most people have heard of and seen clickers in the classroom.

With the proliferation of cell phones and smart mobile devices everywhere, it seems logical that one shouldn't need any other device to take part in an interactive presentation. What Poll Everywhere offers is audience response technology without the hardware. It can be used to poll an audience, for group brainstorming or to allow the audience to add twitter comments to a presentation. Messages can be sent as a text message or online free from any web enabled device.

The basic free service for education in higher ed includes an account with a maximum class size of 32 but an unlimited number of classes. For class sizes up to 400 with mulit-poll reporting, response moderation, and student rosters it's $399 US per semester or $699 US per year.

Here's a link to Use Twitter in PowerPoint