Khan Academy is a nonprofit online learning platform with a wealth of materials in subjects like math, reading and language arts, science, test prep and arts and humanities. It's adaptive, so students can learn at their own pace, and it's got resources for parents who want to help their kids learn.
Another good option is Wolfram|Alpha, a computational knowledge engine that spans a wide range of subjects including math, science, technology, society and culture. It's got the expertise of experts and algorithms to produce answers and perform calculations, and it's good for students and professionals.