Host and Document Collaborative Brainstorming Sessions With Realtime Board - The service allows you to work with any information and visual content on one board individually or with a team. You can work with images, videos, PDF-files, write notes and comment everything, use colorful post-it stickers and files from your Google Drive. The free education version allows you to create unlimited private and public boards, have an unlimited number of collaborators, and 3GB of storage space.
Flockdraw - website states, "is a free to use online whiteboard based painting & drawing tool. It makes it easy to draw online free with multiple people for fun or business. You can have unlimited people in a room and the drawing updates in real time. Uses different colors, shapes and text."