No surprise here, wikipedia tops our list of useful free sites to learn economics. While they do not do a good job of going through examples, or showing the application of the concepts, you can find almost anything related to economics at their site. Another good thing about wikipedia is that the majority of their content has references, so if needed you can look directly at the source.
Investopedia.com has a huge amount of topics including most of the basics, and they take a very textbook styled approach, and you can find almost any topic covered here. They are also a good source because of their sample quizzes that you can take to see if you have mastered the material. Also, if you are a business student, investopedia has other articles on investing and business related topics that you may find useful.
Coming in at number 3 on our best sites for learning economics post is the economics section of tutor2u.net. While this site is difficult to navigate, or find what you need, they have almost everything covered because they have been around so long. There site is formatted similar to this site, in that topics and questions take the form of posts, and are then categorized. This site is especially useful for students in the United Kingdom because the majority of their posts use the UK in their examples and analysis. While the posts are free to peruse, I think they do charge for some of their products.
This site is very similar to number 3. Economicshelp.org has a small section of basic economic principles, and then a very large blog where the author of the site answers questions by the readers, or discusses economic topics of interest. While the brief section on economic principles and the blog posts are free, you are also able to purchase advanced study materials from the site. Again, this site focuses on UK driven economics, so may not be as relevant to those studying in other countries, but still a good source.
Coming in at number 5 on our best sites for learning economics for free is the economics section of about.com. While this site is hard to navigate or find what you are looking for, if you do find it you can be confident that it is well written and representative. And as always, everything on the about.com site is free.
Next we have basiceconomics.info. This site is very straight forward, easy to navigate, and could potentially solve your needs. They do not go over many examples or applications, but have a very brief summary of most basic economic concepts. If you need to find something fast, this site may be your best bet.
Finally I include the economics section of sparknotes.com. Everything here is free and laid out pretty well and they even include a references section. I rate it last because the topics could be better organized and their site is incredibly clustered with advertisements and other items unrelated to studying economics. If you need to kill some time (think facebook), then the off topic items on sparknotes won't bother you, and you should still be to learn what you need to.
So there it is, my favorite 7 sites for learning economics for free on the internet. It is possible that I have missed some, or that you don't agree with my picks so let me know with a comment. Good luck with the semester, remember: C's get degrees! (but F's don't) So study hard!