ChemIDplus National Library of Medicine's dictionary of more than 360,000 chemicals
ChemSpider Free resource from the Royal Society of Chemistry; articles from Royal Society of Chemistry journals and PubMed. Focuses on chemical structure data (does not search articles as thoroughly as SciFinder).
NCBI - National Center for Biotechnology Information - Established in 1988 as a national resource for molecular biology information, NCBI creates public databases, conducts research in computational biology, develops software tools for analyzing genome data, and disseminates biomedical information for the better understanding of molecular processes affecting human health and disease. About NCBI Tools
NIST Chemistry Webbook This site provides thermochemical, thermophysical, and ion energetics data compiled by NIST - National Institute of Standards and Technology - under the Standard Reference Data Program.