"The MIT Media Lab has developed an open source backchannel that would be ideal for the classroom for learners who are for whatever reason unwilling to raise their hands and their voices. […] This tool is easy to set up. You can ask questions, make comments, add links or vote up or vote down a comment. "
The link has more information about this open source tool, with advice on how to use it during a talk (or preferably a series of talks). Good connectivity and power for laptops are required, and a dedicated display recommended, taking a little of the shine off for my regular teaching contexts.
Via Terry Elliott