Padlet is a free service that lets you create online "sticky notes" that you can share and collaborate on. The notes can include images, links and videos, can be moved and arranged and can be embedded into a website.
It's great for brainstorming, story boarding, organizing, projects and more. It is available online and even has a Chrome app and extension, a Kindle app, Safari and Firefox extensions, and a Wordpress plugin.
It's easy to use and fun and very useful.
If you are looking for more features and the ability to have school accounts, privacy, control and more, take a look at Backpack (not free).
More Collaboration Tools and Resources
More Writing Tools and Resources
Slack - free, easy to use, communications/collaboration platform - great for teams and schools
Google for Education Resources