Wiki

There are many free sites where you can set up a Wiki for your class to use. Here is just one example that I created earlier this year with an Ancient History class. Unfortunately they failed to take advantage of it as a learning resource but I think it is a great way for students to share information with their classmates and for teachers to post notes and useful sites.
http://ancienthistory.wetpaint.com/
Or you might like to try
www.stikipad.com