I am a fan of Khan Academy. As a mathematics educator I am often told about people who are struggling or know someone who struggles with mathematics and they are looking to hire a tutor and I ask them to give Khan Academy a shot. I see Khan Academy not as a replacement for teaching but as a way to enhance teaching through a free resource that can bridge home and school, grade to grade, and subject to subject.
Khan Academy started out as a guy making videos for a family member to help explain mathematics concepts and procedures. Now you can not only watch videos but demonstrate competence in those same concepts and procedures through differentiated exercises that are embedded in the site. What is exciting to me is the emphasis that Khan has made on the idea of growth mindset, that we all can learn. That seems pretty simple but how often have we heard people profess their own limitations before they have even tried to do something.
In addition there is way more than just mathematics on the site. I especially like the work they have done in Computer Science and providing entry level exercises so that anyone can learn how to code.