Khan Academy

Khan Academy is a ten-year-old non-profit with a mission of a 'free world-class education for anyone, anywhere'. We're used by millions of learners every month both in the classroom and in their homes. We've got tools for teachers and coaches, plus the official SAT Prep, and a bunch more. I recently blogged about how we're moving from Python 2 to Go: http://engineering.khanacademy.org/posts/goliath.htm. It's a huge and exciting project.