sciencebooksonline.info lists FREE science ebooks, textbooks, lecture notes, etc. You can read the text online or download.
Applications for Science Teachers: This site will be more applicable for secondary science teachers. There are a variety of books organized by content areas such as: biology, chemistry, physics, and more. There is a wide range of titles with a few very interesting old texts. Could be interesting to insert some of these into a class blog as supplemental readings or for research purposes.