Wallwisher.com is a great way to have students work together in a collaborative environment. It does require a log-in, but you can use your Google or Open ID as well as creating an account. There are options that allow comments to be moderated and the owner has control over placement of the comments. Wallwisher's can be embedded into a blogpost as well!
Here is a tutorial I created for students to use wallwisher.com to post a collaborative question.
And the example wallwisher I created