U.S. National Library of Medicine: Chemical Information
Hundreds of thousands of chemicals, plus "synonyms, structures, regulatory list information, and links to other databases." The "ChemIDplus Lite" feature allows searching by chemical name or registry number; with "ChemIDplus Advanced," users "search and display ... registry number, chemical name, molecular formula, structure, physical and toxicological properties plus locator and classification data." Also includes links to additional databases and resources. From the U.S. National Library of Medicine (NLM), National Institutes of Health (NIH).

Go to NLM: Chemical Information