Corkboard.me is a nice alternative to WallWisher. It is totally free, you dont have to sign up to use it, and you create your corkboard just by opening the site. When you visit Corkboard.me, simply copy the URL in your web browser and share it with your friends. They can then add their own notes, links, and images. Corkboards have a near limitless size so they should fit nearly any classroom need.
This is a great resource for checking for understanding in your class, a quick 60 bell ringer to have students voice opinions on a prompt or topic, allowing students to ask questions about a lesson, and much more!
Check it out!