An "Online noticeboard maker," Wallwisher is a pretty nifty tool to get kids thinking about a given topic.
Once the teacher creates the original board, students can access the URL, click a spot on the board and post "sticky notes" of up to 160 characters. They can also embed video, audio or images on the note.
Below is a quick sample I created for a Model United Nations group, imagining I was having them start thinking about some of the current global and diplomatic issues. This could also be used for vocabulary building, as a multimedia presentation tool to break away from Power Point (but easier than Prezi).
Create a site before a difficult lesson, and gather backchat questions as you proceed.
It could also be used as an easy discussion board, with student's posting comments on the night's reading.
For libraries, you could gather student reactions/book reviews and embed the wall on your library site.
Teachers can opt to moderate comments before they are posted, always a good feature!