iTunes U. The "U" stands for University, is a place where dozens of the world's top universities make thousands of lectures available online. Most of them are completely free, and can be accessed by anyone, regardless of whether you're a student at the university or not.
The lectures cover every subject under the sun, and because they were produced within leading universities it's a fair bet that they're accurate and trustworthy.
To use iTunes U, the only downside is that you'll need to install iTunes on your PC and sign up for a free account. The download, from http://www.apple.com/itunes, is around 75 MB and is not the easiest to use with a screen reader. Once you're done, head to the iTunes Store and then click on the iTunes U button in the top menu bar. You'll be glad you did.