A free, open collection of college course material hosted by Saylor Foundation. It is designed to replicate an entire college curriculum with freely available course materials, although they do not offer college credit. (Eventually they will to host all materials on their own site so they can ensure long-term stewardship, but in currently some is available only through links to other sites). Currently the courses consist largely of recorded lectures and readings. There are a few quizzes, but few interactive...
Serendip is an independent site partnering with faculty at multiple colleges and universities around the world. Happy exploring!
Blended Learning Resources
Website of the Exploratorium: Museum of Science, Art and Human Perception in San Francisco. No courseware, but links to some fabulous multimedia and interactive features and offline "things to make and do" activities that the museum has developed on a wide range of topics related to math, science, art, and human understanding. Elementary school to adults. The Exploratorium also offers two apps, "Sound Uncovered" and "Color Uncovered," which are free, interactive iPad books.