TodaysMeet is a free, easy-to-use backchannel.  I first learned about TodaysMeet at a workshop where we were asked to post questions during the presentation; rather than disrupting speakers, those questions were addressed during breaks.

No registration is required.  Students can participate with any device that connects to the internet (laptop, tablet, phone).  Just create a room by giving it a name, and direct students to the easy-to-remember address provided.  You can set rooms to be open for anywhere from two hours to one year.  Students do not need to register, and you can even keep it anonymous.  While the anonymity option can be seen as a potential drawback, I have used it to encourage participation from shy students.

I recently used TodaysMeet with my students to ask them to explain the difference between caribou and reindeer.  The advantage of using a backchannel is that you get the immediacy of a verbal response, but make it easy for even quiet students to have a voice.  Try leaving one up during class lectures to increase questions, comments, and overall participation.

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All responses can be archived.  For example, I took the transcript from a session I facilitated at an Ed Camp and saved it as a PDF.  I then saved the PDF in Dropbox and shared a link to the file with all participants, and on Twitter.  That particular session was rich with comments, questions and even shared links that I have referred to many times since.  I doubt such a take-away would have been possible without a backchannel.