Todar's Online Textbook of Bacteriology: college or "advanced high school" level (Kenneth Todar of UWisconsin-Madison)
CK-12 Foundation provides a library of free online teacher resources, including textbooks, worksheets, group activities and more, covering more than 5,000 math and science concepts from arithmetic to chemistry
Bookboon's free online textbooks for students are focused and to the point. They are all written by highly respected professors from top universities in the world and cover topics such as economics, statistics, IT, engineering and natural science.
Calculus for Beginners and Artists is an online textbook that provides an overview of Calculus in clear, easy to understand language designed for the non-mathematician. Thank you MIT OpenCourseWare for awesome, free resources and classes!