Stormboard is a free tool for real-time collaboration, brainstorming, and mind-mapping. Using Stormboard, users can create an unlimited amount of "storms" from a bank of education templates (Character Map, Compare & Contrast, Conflict Map, Reading & Analyzing Non-Fiction, etc.) or start from scratch with a blank brainstorming wall. Once the storm is created, users can invite up to 5 collaborators via email, providing the link to the storm, or giving out the storm ID & key.
Your storm can include color-coded sticky notes, images, videos, and sketches. Users can utilize the chat option to communicate within the storm. The drag-and-drop process makes it very easy to edit the storm. A couple of the nice features of Stormboard is the ability to vote on the ideas on the storm (each user gets 10 votes to prioritize the sticky notes) and the option of commenting on each item on the storm so that your collaborators can give feedback on the ideas. You can retrieve reports from your Stormboard in a variety of formats.
The Google sign-in feature makes this an easily accessible tool for your Chromebook users. The numerous literacy templates are great if you're thinking about going paperless.
*Free education accounts until July 31, 2014 which enables unlimited users per storm*