I have been on the outlook for another cool collaborative tool, one that requires a very low learning curve so that students can just jump in and work with it.
Today I finally found a moment to look through my Diigo groups and came across, among many things, Spring Note.
Spring Note is a wikish sort of tool, that is set up with templates for to do lists, members, discussion, and links. Now, you could easily set up a wiki template to do the same job, but SpringNote has already done this for you. The options vary depending on the type of notebook you wish to create.
It comes with a nice WYSIWYG editor which allows you to embed code for maps, video and slide shows and more. It certainly is worth a go if you are planning on engaging in work of a collaborative nature.