http://physicsforums.com/
P.S. to get the best of the e-learning experience, one must have valuable sources such as Wikipedia, but the ability to discuss the material with others on places such as a forum like this.
If enough people understood the e-learning process, we would have many more educated minds without the expense of shelling out cash. I think some people here with me will agree that the reason people contribute to the forum is so people can get more intelligent, and while on their journey help others too.