Wallwisher (now known as Padlet) was another old familiar tool I came across recently. I stopped using it as it was not always reliable, but it has had a revamp and seems to be working quite well. What I especially like about it is that it also works on iPads, iPhones and iPod Touches. It is not an app but you can access it through the Safari app by going to http://padlet.com/ if you are starting a new one, the URL that designates the Padlet you have created or as a QR code (try out the one to the right) which is generated for you within Padlet. You do need to set up an account to create a Padlet but your students do not need logins or accounts to access it.
You can add text, graphics, movies, maps, slideshows, documents and photos from webcams.
You can embed the Padlet in a wiki or or a blog.
Below is a Padlet wall that I have had for a few years with some ideas of how to use Padlets in the classroom. Please feel free to add to it.