Some of you may already know Khan Academy, but for those who don't here are the cliffnotes. Khan Academy is basically a collection of YouTube videos which help you learn algebra, geometry, physics,... It was originally made as a teaching aid by Salman Khan for his cousins, but it has grown into something quite a bit bigger.
I'm currently going through the chemistry course, and it's really enjoyable. So if you feel like learning something new, or maybe just taking a refresher course, Khan Academy is there for you.