ChemBioFinder Free but you must register. Enter a Chemical Name, CAS Number, Molecular Formula or Weight. Use * for partial names (e.g. ben*). Includes boiling and melting points, specific gravities, evaporation rates, vapor densities, water solubilities, and EPA codes. Also, includes molecular or structural images of the substances.
Chemistry & Chemicals @ Science.gov Has a number of important chemistry-related databases, including ChemWebBook, CHEMIDPlus, IRIS, CancerNet. etc. Many are searchable by compound name, CAS Registry Number, structure, etc.
ChemSynthesis "Contains substances with their synthesis references and physical properties such as melting point, boiling point and density. There are currently more than 45,000 synthesis references in the database."
MatWeb A searchable database of material properties.
NIST Chemical Kinetics Database The NIST Chemical Kinetics Database includes all reported kinetics results for thermal gas-phase chemical reactions. The database is designed to be searched for kinetics data based on the specific reactants involved, for reactions resulting in specified products, for all the reactions of a particular species, or for various combinations of these.
Organic Chemistry Portal Created by industry chemist Reto Müller of Switzerland, this has a database of abstracts from eight organic chemistry journals, information on over than 100,000 compounds, and more.
Organic Syntheses "Since 1921, Organic Syntheses has provided the chemistry community with annual collections of detailed, reliable, and carefully checked procedures for the synthesis of organic compounds."
PubChem Project A component of NIH's Molecular Libraries Roadmap Initiative, PubChem includes substance information, compound structures, and bioactivity data in three primary databases, PCSubstance, PCCompound, and PCBioAssay.
Reciprocal Web A collection of Common Molecules that presents general and crystallographic information on compounds. Innovative visualizing tools provide a valuable glimpse into structural details of those substances. Advanced tools and crystallographic data are available.
SOLV-DB A database from the National Center for Manufacturing Sciences containing a wide variety of data on solvents.
Spectral Database or Organic Compounds "SDBS is an integrated spectral database system for organic compounds,which includes 6 different types of spectra under a directory of the compounds."
Synthetic Pages Interactive database of synthetic chemistry. Organic, organometallic and inorganic chemical synthesis, reactions and procedures, as well as the synthetic methods on the site are updated continuously by chemists working in academic and industrial research.