Physical and Chemical Properties

Sigma-Aldrich
Lists common physical properties (boiling point, molecular weight, etc.) for chemical compounds that are available through the company.

Merck Index
Encyclopedia of chemicals, drugs, and biologicals that contains more than 11,000 monographs. Each monograph in this authoritative reference source is a concise description of a single substance or a small group of closely related compounds.

Knovel Scientific and Engineering Databases
Provides information on a number of chemical and physical properties for chemicals, esp. industrial chemicals. The interative tables and interactive graphs features also allow easy ways to manipulate data.

NIST Chemistry Webbook
Provides a number of physical properties such as thermochemical, thermophysical, and ion energetics data compiled by NIST under the Standard Reference Data Program.

ChemSpider
Provides access to experimental and predicted chemical properties data from hundreds of sources for millions of structures.

SpringerMaterials
Encompasses all volumes of Landolt-Börnstein New Series (largest and most respected compilation of data in physics and chemistry), the complete Linus Pauling Files (properties of inorganic solid phases), a subset of the Dortmund Database of Software and Separation Technology (thermophysical properties of pure liquids and binary mixtures), and other specialized databases.