Posterous They call it a blog but it is the easiest way to post content I've found yet. If you can email, you can post.
Google spreadsheets A spreadsheet many can work on at the same time
Voicethread - Tool to allow people to comment on a document, photo, video or other type of media in 5 different ways.An example of a Voicethread
Wikipedia (if you want to try editing the granddaddy of all wiki's, here's the Wikipedia sandbox)
YouTube EDU - a source of educational videos you can use in your courses