PubChemU.S. National Library of Medicine (NLM), National Center for Biotechnology Information
--An open chemistry database which mostly contains small molecules, but also larger molecules such as nucleotides, carbohydrates, lipids, peptides, and chemically-modified macromolecules, collecting information on chemical structures, identifiers, chemical and physical properties, biological activities, patents, health, safety, toxicity data, and many others.