Knovel - A collection of engineering handbooks and references sources in the following subject areas: civil engineering & construction materials, electrical & power engineering, mechanics & mechanical engineering. Features include full text searching and interactive tables and graphs.
CAMEO Chemcials (NOAA.gov)
Chemical and physical property data compiled by the National Oceanic and Atmospheric Administration
Chemistry WebBook (NIST.gov)
Chemical and physical property data compiled by NIST under the Standard Reference Data Program.
PubChem Project (NIH.gov)
Hosted by the National Institutes of Health. Provides incredible wealth of information on the material. Results include safety/toxicity, literature links, physical data, and more.
Material property data on over 28,000 metals, plastics, ceramics, and composites.