There are a number of free educational resources on the web. Many of them are very good or even excellent. Today, while looking for a review of "degrees of freedom", I found the Online Statistics Book, a project that was "partially funded by the National Science Foundation". It's free, or what's known as an "Open Educational Resource (OER)". No surprise there, as the National Science Foundation has funded other similar publications that are freely available on the web. This particular book is described as follows: "Online Statistics: An Interactive Multimedia Course of Study is an introductory-level statistics book. The material is presented both as a standard textbook and as a multimedia presentation. The book features interactive demonstrations and simulations, case studies, and an analysis lab."
A list of contributors, and the universities that helped with the development of the project, as listed on the homepage. There is also a comprehensive table of contents, a link to Rice University's Virtual Lab, and a list of "simulations and demonstrations". Additionally, the book is downloadable, so it can be read/used offline.