Khan Academy

With a video library of over 3,100 videos on everything from arithmetic to physics, finance, and history and hundreds of skills to practice, we're on a mission to help you learn what you want, when you want, at your own pace.

From Salman Khan's FAQ: My uncle's family visited me in Boston after my wedding in the summer of 2004. At some point during the trip, my Aunt told me that her daughter (my cousin) was having trouble with "unit conversion" which was not allowing her to be placed in the more advanced math track for 7th grade. Nadia was clearly a very bright girl, so I made a deal with her. I'd remotely tutor her for an hour after work as long as she was willing to do any extra work I gave her.

I began remotely tutoring Nadia in August of 2004. She was in New Orleans--where I also grew up-- so we used a telephone to talk and Yahoo Doodle as a shared notepad. Nadia ended up catching up and getting ahead of her class so I started tutoring her brothers, Arman and Ali, as well. Eventually, word got around and I was remotely tutoring and handful of cousins and family friends. Scheduling around my work, their soccer practice, and the different time zones became a little ridiculous, so I started to make YouTube videos for them to watch in their own time, at their own pace.

It didn't take long to see that other students (including adult learners) were hungry for videos like these so I kept going!

Even before I made the videos, I started writing simple Javascript problem generators so that my cousins would never run out of practice problems. I wanted to know when and how they were doing the problems, so I added a database to track usage. 70 modules and 10,000 lines of code later (much of which has made the software adaptive), it has morphed into the adaptive assessment exercises on our site.

Publicly available: yes
Countries where available: Worldwide
Price range (USD): Free
Available online: yes