If you are looking for physical or chemical property data about a substance or material, here are some good starting points:
See also:
Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals
Print book available in Chemistry Library (RS51 .M4 2013)
A comprehensive reference work on organic chemistry that provides information on thousands of substances and structural formulas.
Looking for an article or trying to verify a citation?
Databases included: CAplus (Chemical Abstracts), REGISTRY, CASREACT (chemical reactions database), CHEMCATS (commercially available chemical information), CHEMLIST (regulated chemical information), and MEDLINE (biomedical literature worldwide).
Coverage:
Looking for syntheses of or information about organic or inorganic chemical substances?
Note: SciFinder and Reaxys are complementary resources, and although there is some overlap the two are quite different in their respective literature coverage, and the ways they register compounds and index reactions. It's advisable to consult both databases for the most complete coverage of compounds, reactions, and properties.