Wallwisher (stickies on the wall)
The digital/online version of writing comments on a Post-It note and sticking it to the wall.
Walls can be private or public. You (your students) don't need an account to post on a public (course) wall.
Kwantlen Learning Technology wall http://www.wallwisher.com/wall/kwantlen
June 2011: "We would like to remind you that the service is still in beta and so you might encounter some problems. Please note that the wall page only supports browsers IE7+, Firefox 2+, Safari 3+, Chrome 1+, and Opera 9+. Feel free to contact us by emailing us at email@example.com if you run into any problems."