From the Royal Society for Chemistry, ChemSpider is a free chemical structure database providing fast access to over 40 million structures, properties, and associated information. By integrating and linking compounds from ~500 data sources, ChemSpider enables researchers to discover the most comprehensive view of freely available chemical data from a single online search.
PubChem provides information on the biological activities of small molecules. PubChem is organized as three linked databases: PubChem Substance, PubChem Compound, and PubChem BioAssay. PubChem also provides a fast chemical structure similarity search tool.