PubChem
Provides access to information on the biological activities of small molecules. It is organized as three linked databases within the National Center for Biotechnology Information (NCBI) Entrez Information retrieval system: PubChem Substance: Search chemical substance records using names, synonyms, or keywords.
PubChem Compound: Search chemical compounds using names, synonyms, or keywords.
PubChem BioAssay: Search via terms one would expect to find in the description of a bioassay of interest.