4. Khan Academy
Users can browse 10- to 15-minute videos showing creator Salman Khan and others doodling away on a digital black board to explain subjects from making hexaflexagons to the stereochemistry of dienophile.
Khan Academy also features new videos that help explain issues the world faces today – like a four-part series on the Greek debt crisis or the introduction to the US social security system.
Khan Academy’s videos hosted on YouTube have received more than 200 million hits.