Knotbooks is a very cool service that allows users to create, collaborate and share customized, self-guided physics lessons. The goal of the site is to" make all of physics accessible, understandable, interactive, and free".
The site is free to use and you can either create an account or login using a variety of other accounts, such as Facebook, OpenID, Myspace, WordPress, Google, Twitter, and many others.
You can create self-guided multimedia lessons, study guides, and other resources using a huge resource of knowledge and information from their partners. You can also modify existing Knotebooks for your own use and collaborate and get help on any topic in physics from the community and resources. Text, animations, videos, and much more are all a part of Knotebooks.
While it is only for Physics, I see this as a great model to use in all topics.