ChemSpider is a free chemical structure database providing fast text and structure search access to over 57 million structures from hundreds of data sources. Provided by the Royal Society of Chemistry.
PubChem is a NLM database of compounds, substances, and bioassays, focusing on the biological activities of small molecules. PubChem provides substance information, compound structures, and bioactivity data for small molecules in three primary databases, PCsubstance, PCcompound, and PCbioassay. PubChem also includes a chemical structure similarity search tool.
Toxnet is a group of databases covering chemicals and drugs, diseases and the environment, and toxicology. Information found in the TOXNET databases covers specific chemicals, mixtures, and products; chemical nomenclature; and special toxic effects of chemicals in humans and/or animal. Citations from the scientific literature. It is managed by the the National Library of Medicine (NLM).