Backchannels are becoming quite popular. They are useful, too, for the quiet kids, who are generally reluctant to talk: This is a site where students can post questions, take a preset quiz, or an impromptu quiz, or vote on ideas or discussion points. It's a great tool for teachers to get a quick understanding of whether or not the class has gotten the point of a certain lecture, class or concept.

These are sites that allow for polling, to get a quick idea of how many students favor one idea over another. This is a great visual site for getting a quick overview of students' understanding of a lecture or a concept.
Both of these sites are like virtual chat rooms. Todaysmeet is a great, free resource for allowing kids who are generally quiet to have their voices heard. A virtual corkboard, where people can share ideas, pictures, website, documents, almost anything that someone would like to share. This is a virtual binder that does much the same thing as but organizes the information like a binder.