University level courses from the top schools around the world, online! 

https://www.edx.org/

Adult Life-long Learning:  K - 12 courses in all subjects: 

www.thegreatcoursesplus.com

Online Courses for Children and Young Adults in all different languages / Online Kurse für Kinder und Jugendlichen in allen verschiedenen Sprachen / 给青少年和孩子们的多语言网课。 

https://www.khanacademy.org/

https://zh.khanacademy.org/