Merck Manual for Professionals - First published in 1899, the Manual is one of the most widely used comprehensive medical resources for professionals and consumers, with concise information in everyday language on health topics, symptoms, emergencies, drug information with pill identifier, news & commentary, and other resources on common medical tests, medical terms, abbreviations, images and more!
PDR - Prescriber's Digital Reference provides search and browse for over 2,300 drug guides by drug name.
Free Books 4 Doctors - anyone can read the 365 medical texts, 38 of which are in Spanish.
National Academies Press - read more than 3,000 books free online; purchase chapters in PDF format
National Center for Biotechnology Information Medical Bookshelf - The Bookshelf is a growing collection of biomedical books that can be searched directly by typing a concept into the textbox. From NCBI with links to abstracts in PubMed