PubChem is a database of chemical molecules and their activities against biological assays. It's maintained by the National Center for Biotechnology Information (NCBI).
This National Library of Medicine's (NLM) database of over 400,000 chemicals can be searched by chemical name, CAS Registry Number, molecular formula, and structure or substructure.
ChemSpider is a free chemical structure database providing fast text and structure search access to over 58 million structures from hundreds of data sources. It's provided by Royal Society of Chemistry (UK).