The Textbook Revolution archive is the largest collection of free textbooks on the web. What could be wrong with that? Well, the bad side is that some of these great additions don’t get all the attention they deserve.
For instance, economics students or professionals needing to brush up on some concepts should visit this large link-list from the economics dept at Oswego. In addition to some good overview materials there are some advanced materials focusing in on topics like econometrics and money mechanics.
Speaking of economics, if you haven’t seen Prof. McAfee’s Intro Econ textbook. This is one of the best free textbooks available - large, complete (and did we say free?).