Science & Tech
Association for Computer Machinery Digital Library
Light and Matter
O'Reilly Open Books Project. Note on this one, some of the links lead to semi-broken pages, but if you scroll down, the books are broken down into pdfs of each chapter.
OpenBookProject
FreeTechBooks. This one may jump out as fake, but in reality it's a collection of links to original sources of freely distributed books, e.g. A Practical Introduction to Python Programming.
Philosophy
Stanford Encyclopedia of Philosophy
General Info
GCFGlobal