1. Khan Academy
A site created by American Educator and Computer Enthusiast Salman Khan. Teaches Science, Math, CS, Economics, Finance and also commercializes test-prep all for free.
A site created by several American programmers built in Ruby on Rails. Teaches Computer Science courses in languages like Ruby, Python, HTML, Java, AngularJS and so on for free, and offers quiz, tests and mock tests for a paid account.
An Indian Site developed for CBSE and ICSE boards that forms the base of Indian Social Learning.
An online learning portal powered by many universities.
A Polish Multinational firm that forms a social learning base in the cloud.
Hope it helped!!!