Math Open Reference
A free online interactive geometry textbook. The site has two primary audiences: teachers using the animations in class with a projector to demonstrate geometry concepts, and students who need a reference source when the teacher is not around.
The scope of the site is all geometry concepts up to and including high school and college general ed.
Each topic has a java applet that both animates the concepts and allows the user to interact with it to gain a deeper understanding. The idea is to go beyong what a static paper textbook can offer, and make the content more available without carrying heavy books everywhere.