I originally reviewed Math Open Reference last year. In the time since I wrote that review there have been some additions made to it. Therefore, I’m bringing it to your attention again or perhaps for the first time.
Math Open Reference is a free online reference for geometry teachers and students. Math Open Reference features animated and interactive drawings to demonstrate geometry terms and concepts. The table of contents on Math Open Reference is divided into four basic categories; plane geometry, coordinate geometry, solid geometry, and function explorer tools. Click on any subject in the first three categories to find definitions, examples, and interactive drawings. In the function explorer category users can select linear functions, quadratic functions, or cubic functions to explore how changes in variables affect the graphed output.
Math Open Reference probably still isn’t complete enough to replace a textbook, but it could make a great supplement to the mathematics textbooks that you do use. For students who need visual references, Math Open Reference could be particularly helpful.