From the National Institute of Standards & Technology, contains thermochemical and spectra data for thousands of compounds. You can search on name, chemical formula, CAS registry number, molecular weight, chemical structure, or selected ion energetics and spectral properties.
A database maintained at the National Library of Medicine on over 380,000 chemicals. Basic physical and toxicological data is given, as well as links to government and regulatory information. Can be search by name, properties, or chemical structure.
ChemSpider is a chemistry search engine. It has been built with the intention of aggregating and indexing chemical structures and their associated information into a single searchable repository and make it available to everybody, at no charge.
A database of basic reaction types. You can look up a molecule or type of chemical (e.g. ketones) and see what kind of reactions they participate in or how they are formed. You can also browse a list of common reaction mechanism.
The ChemWiki project is a collaborative approach toward chemistry education where an Open Access textbook environment is constantly being written and re-written partly by students and partly by faculty members resulting in a free Chemistry textbook to supplement conventional paper-based books.
Part of ChemOffice, from CambridgeSoft. The University of Toledo has a site license for this software, and you can request a download link at this site but you must use your university e-mail account to do so.