ChemSpider: ChemSpider is a free chemical structure database providing fast access to over 57 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. It is owned by the Royal Society of Chemistry

NIST Chemistry WebBook National Institute of Standards and Technology. Search by formula, name, IUPAC identifier, CAS registry number, and more.

PubChem: PubChem, released in 2004, provides information on the biological activities of small molecules. PubChem is organized as three linked databases within the NCBI's Entrez information retrieval system. These are PubChem Substance, PubChem Compound, and PubChem BioAssay. PubChem also provides a fast chemical structure similarity search tool. More information about using each component database may be found using the links in the PubChem homepage. Links from PubChem's chemical structure records to other Entrez databases provide information on biological properties. These include links to PubMed scientific literature and NCBI's protein 3D structure resource. Links to PubChem's bioassay database present the results of biological screening. Links to depositor web sites provide further information.