ChemIDplus - An online dictionary of over 370,000 chemicals maintained by the United States National Library of Medicine. Includes chemical structure, properties, and toxicity information.
ChemSpider - This free, searchable database contains chemical structure information for over 26 million structures. It lets you search by systematic names, synonyms, trade names, and database identifiers.
Dynamic Periodic Table - A free, interactive periodic table that features properties information for each element, including its state at O degrees Kelvin, melting point, boiling point, and electronegativity.
General Chemistry Online - Maintained by Frostburg State University, this website provides a searchable database of common chemical compounds, tutorials, and a glossary of important terms.
MSDS Online - This website maintains a comprehensive list of places you can find free material safety data sheets online. It also features a glossary, FAQ, and links to useful chemical safety information.
Periodic Table of Videos - Maintained by the University of Nottingham, this website provides educational streaming videos for each element in the periodic table.
The Wired Chemist - A collection of online chemistry resources, featuring animations, learning modules, and laboratory tutorials for chemistry students.