ChemIDplus indexes more than 368,000 records for chemical substances cited in the National Library of Medicine (NLM) databases. It is searchable by Name, Synonym, CAS Registry Number, Molecular Formula, Classification Code, Locator Code, and Structure (requires Chime plug-in or ISIS/Draw). The records contain information on structure, names and synonyms, formulas, registry numbers, and some physical properties in addition to toxicity information and links to NLM-indexed articles which cite the substances.

http://chem.sis.nlm.nih.gov/chemidplus/