Timothy Taylor, one of my favorite economists, has a textbook that you can read for free, provided you are willing to put up with registering at the site and having some of the pages be ads. I thought that the hassle was minimal. Go to this web site to register, then search for Taylor's economics principles textbook.
It appears to be a fairly standard, middle-of-the road text. I only looked at the table of contents and a few chapters.
My guess is that for a free online reference, the best thing might be something like the concise encyclopedia of economics, where you can look up terms as you need them. But the CEE does not have the diagrams that you can find in textbooks. For those, an online textbook like Taylor's might be handy.