About.com -- If you or someone you know has a curiosity for how the World works this free textbook is something worth checking out. The free textbook is written in an entertaining and understandable style, yet manages to teach the concepts of classical and modern physics. The book is a 1200 page 40 megabyte PDF file and is fully searchable. The textbook is written to challenge and surprise the reader on every page. I wish there had been such a great free reference when I was a budding young scientist.