Yes. Khan academy probably the best online academy. They have got so many free youtube videos. I have studied from those videos and aquired good marks.
No doubt one can critisize khan academy. They have got well experienced teachers, and their teaching skills are just excellent. If you want to spend money on online teaching, then go for Khan Academy. You are at the right place. Dont worry at all.
Do look into their free youtube channel as well. There are lot o content in it.
Khan academy not only teaches at the school level but also for the college students.