Mathematics and Calculus: Top 5

Looks at the language of numbers through common situations, such as playing games or cooking. Puts your decision-making skills to the test by helping you decide whether buying or leasing a new car is right for you, and by enabling you to predict how much money you can save for your retirement by using an interest calculator.

Non-profit organization dedicated to "providing a free world-class education to anyone anywhere." The materials and resources on the website are available free of charge. There are over 3,100 videos on everything from arithmetic to physics, finance, and history, with hundreds of skills to practice.