"ChemSpider is a free chemical structure database providing fast text and structure search access to over 40 million structures from hundreds of data sources." From the Royal Society of Chemistry (UK).
"Chemical database is a dictionary of over 400,000 chemicals (names, synonyms, and structures). ChemIDplus includes links to NLM and other databases and resources, including links to federal, state and international agencies." From the U.S. National Library of Medicine.
"The Sheffield Chemdex: the directory of chemistry on the WWW since 1993." Search or browse categories: Chemistry Link Type (Elements & compounds, Chemistry departments, etc.), Countries, and Chemistry (Organic compounds, Biographies, etc.)