Online Classes has a great article entitled "100 Free Tools to Tutor Yourself in Anything". Online Classes is a site that mainly deals with online schools and programs, but they also have resources like this article.
The article has a list of 100 web sites and resources on different topics. The topic categories range from "general" to "language" to "science" to "business" and more. Many of the links will be familiar to most people (NASA, Google) but many are a little more obscure.
I really found the list to have a lot of great links and resources.