Khan Academy is an amazing collection of over 2,600 videos and practical exercises covering everything from math to science to the humanities. And it's all free! Learn what you want, when you want, at your own pace. Here's what Khan Academy says about itself (from the About page on their website):
A free world-class education for anyone anywhere.
The Khan Academy is an organization on a mission. We're a not-for-profit with the goal of changing education for the better by providing a free world-class education to anyone anywhere.
All of the site's resources are available to anyone. It doesn't matter if you are a student, teacher, home-schooler, principal, adult returning to the classroom after 20 years, or a friendly alien just trying to get a leg up in earthly biology. The Khan Academy's materials and resources are available to you completely free of charge.