Wikis are scary.
This year, my juniors are creating a wiki for the novel Song of Solomon--character descriptions, thematic analysis, quotable quotes, historical background. Peppered with links, and editable at any time by anyone in my classes.
It's a nice extension of habits we've already developed by blogging. Now, though, information and analysis are no longer the sole possession of individuals. They belong to the community, with all the attendant ethical obligations. That sort of power is, as Lord Acton observed, eminently corruptable, so I've laid down strict expectations about malfeasance and stupidism, and won't hesitate to ban anyone who practices either. I hope that the students can largely police themselves.
I'm using wikispaces because it's free and intuitive, with very few idiosyncracies. (One is a lack of drag-and-drop linkage.) For the teacher who's willing to take the pedagogical and technological risks, the rewards are ample: engagement, cooperation, and even fun.