While browsing Early Modern Notes, I came across some links to the Historical Text Archive. This archive contains a solid collection of articles(682), ebooks(69), and links(6036). You may noticed that it is hard to find good free history textbooks and materials. The Historical Text Archive won't solve that problem alone, but it is a good start.
Also, if you are interested in the early modern (c.1500-1800) be sure to check out Early Modern Notes. Sharon's blog includes many helpful links and "latest resources" such as this interesting site from Columbia comparing the development of China and Europe from 1500-2000.