Wallwisher.com is a great site for creating an online bulletin board.  Boards can easily be created by posting sticky text notices, photos, weblinks, and more.  Creators of the wall can either keep their wall private (for todo lists and personal notes) or invite others to view and collaborate with the wall by posting their own sticky notes.

I can see a lot of options for students using this as a collaboration tool for projects and studying.  So far, we have created a page for interesting weblinks where students are encouraged to find and post interesting links that pertain to the class subject matter (and fall within school internet use guidelines).  When school starts back, I will be creating online word walls for key terms with our current lesson units.