Twiddla is a great free tool that allows students to share a collaborative whiteboard without having to download an extension or log in to a service.  Through Twiddla, students can also navigate through and annotate webpages to collaborate on projects even if they’re not in the same location.  Twiddla offers a few premium features like the ability to store images on their server and password protect meeting sessions, but most circumstances won’t require a subscription.

Website: http://www.twiddla.com/
Subject: All subjects
Grade Level: K-12
Price: Free