I love e-learning I do lot of free courses online from different websites like coursera, edx, khanacademy, udacity etc.I feel that different website has got different courses that they are expert at there is no one website which provides all.
About the Website: The khan academy is founded by salman khan who is graduated from MIT and from his Ted talks I came across this website that his motto is to provide a free, world-class education for anyone, anywhere.
I did a course called cryptography from this website it had different stages starting from history of cryptography, cipher, modern cryptography, randomized algorithms etc which is pretty detailed.
This website is very useful for everyone their are courses from different fields and is also provides great foundation for kids with math etc.