Properties of Substances

This is a good source of thermodynamic data.

This is a database of IR, NMR and Raman Spectra from the National Institute of Advanced Industrial Science and Technology of Japan. Includes some inorganic compounds as well.

High precision isotope masses.

NIST web site which has information useful for comparing computational methods. Includes geometries and some other molecular properties.

A site maintained by the American Chemical Society that tells what is in many common consumer products.