Writeboard's new, free collaborative writing software will be of interest to teachers and their students. It's basically an online paperboard which one or many users can use to work together on a group project. It allows easy text formatting, hyperlinks, headings, lists and images.
It could be used to draft texts, write a collaborative poem, for groupwork...the possibilities are numerous. Setting up a writeboard is extremely simple (just an e-mail and password required), you can make as many as you like and it's free.