NASA has a huge library of educational material for grades kindergarten through college dealing with many aspects of space travel and physics. On their education page, they not only have lesson plans for teachers, but educational games, activities and homework help for children of all ages visiting the site.
NASA also maintains a website for their Jet Propulsion Lab at the California Institute of Technology. Here you can find lesson plans, activities and downloadables that cover weather, rockets and more information about space for use with students of all ages.
Sea World has a large collection of animal and conversation educational resources on the web for both teachers and students. Students can read about a large number of animals in both the online Animal Byte fact sheets and online books. For teachers, there are downloadable teaching guides, activities and ShamuTV.
The Massachusetts Institute of Technology offers 2,000 past classes from previous years in almost every subject, complete with textbooks, online as open courseware. These are all college-level classes that will satisfy homeschooling requirements in most states and are suitable for high school or more advanced middle school students.
The Reading A-Z website offers everything you need to teach reading. Resources include books that you can download along with reading comprehension worksheets for those books, phonics worksheets, resources for teaching the alphabet, sight word books and much more. The resources are directed to early elementary students, but could easily be used with older students with learning disabilities.
This website was created by the Independence Hall Association in Philadelphia. UShistory.org offers an online textbook that covers the pre-Columbian period through today. Sections of the site pay individual attention to things like The Declaration of Independence and the Liberty Bell.
Free-ed.net offers a variety of basic classes, from general math to auto repair, free of charge. Some of the classes are taught right on their page, for others, they redirect you to another website for the class. Many of these courses are taught via video.