This site is great if your child is having trouble understanding a concept. The obviously educationally gifted creator of his site has recorded hundreds of videos on math(where he started) and even science and humanities. There is also a diagnostic
Uc Open Access
Hippocampus has numerous AP level courses online. The courses are well done and great for self learners. After using this course, students can feel confident taking AP and even CLEP exams. UC Open Access is very similar and designed by the same people, but the courses are organized a little differently, and the tests are different. You might want to check out both to decide what is your preference. My daughter prefers Hippocampus for science and social studies and UcOpenAccess for math.
These ebooks include numerous math titles, language arts, science, GED prep and more. I found these especially helpful in reviewing subjects that public school kids had failed and also for quick summer courses (my son used the Trig book which was not free, but worth the $5.00 I spent on it times 10.)
After spending hundreds on Rosetta Stone only to have problems installing it and then having another child refuse to use it, I found numerous free online Spanish (and other language) curriculum that were quite good. They include: Destinos from learner.org, BYKI, and BBC Spanish.
Simply Charlotte Mason
Simply Charlotte Mason is the best bet for younger children. Children learn primarily through reading great "living books" which helps them to more fully experience education. It includes all subjects for all grades, K-12.
Included (in the pictures) are my favorite reference books for homeschooling.