Very similar to Wallwisher. Great for class discussions and sharing ideas. Could be used just like a forum but I like to layout where you can see all posts at once and read the most relevant first. Double click on the posts to enlage. Have a go on the noticeboard below.
I found it a little trickier to get the hang of but it does so much more. Instead of just text you can post pictures, files, hyperlinks. You can even add a poll as a post giving readers multiple choice options (try voting on mine below!) I think my Year 6's would get the hang of it pretty quick but for lower KS2/KS1 Wallwisher is probabily easier. To upload files or pictures you need to be logged in but setting up a class account gets round this.
What's really useful is the "How to..." guides on YouTube - really useful for getting started. It's really easy to embed into your website/learning platform, either as a small widget board as I've done here or as a full page board.
NOTE: There are "chat" and "who's online" options but these are easy to disable when you set up the wall under "Tools".