Coarkboard.me is another way to use a collaborative web space in your classroom. It does not require a log-in, and is a fairly basic coarkboard system. Students can create stickies of various colors that can be resized easily. It also allows you to post pictures to the board by copying and pasting the image URL into a sticky note. This will turn the sticky note into a polaroid snapshot of the image.
Here is a tutorial on the basics of using coarkboard.me in the classroom
And an example coarkboard