Databases

Bulk Density Chart
Densities in standard and metric form, grouped by first letter of the product.
http://www.asiinstr.com/technical/Material_Bulk_Density_Chart_A.htm

Dielectric Constant Reference Guide
Single page of several hundred chemicals, sorted by name, with associated dielectric constant.
http://www.asiinstr.com/technical/Dielectric%20Constants.htm

EMSL Gaussian Basis Sets
Extensible Computational Chemistry Environment Basis Set Database, Version 8/01/01, as developed and distributed by the Molecular Science Computing Facility, Environmental and Molecular Sciences Laboratory which is part of the Pacific Northwest Laboratory.
http://www.emsl.pnl.gov/forms/basisform.html

Fundamental Physical Constants from NIST
The values of the fundamental physical constants provided at this site are recommended for international use by CODATA and are the latest available.
http://physics.nist.gov/cuu/Constants/

GRI-Mech
site at Berkeley also has selected kinetic rate constants and parameters for reactions involving the species included in their database.
http://www.me.berkeley.edu/gri_mech/data/rxns_static.html

JPL Molecular Spectroscopy
Database which has an extensive library including programs for fitting spectra and spectral line files for a large number of molecules. Use requires knowledge of spectroscopy and quantum mechanics.
http://spec.jpl.nasa.gov/

National Institute of Standards and Technology WebBook
Thermochemical data for over 6000 organic and small inorganic compounds. Reaction thermochemistry data for over 9000 reactions. IR spectra for over 8700 compounds. Mass spectra for over 12,000 compounds. UV/Vis spectra for over 400 compounds. Electronic and vibrational spectra for over 4000 compounds. Constants of diatomic molecules (spectroscopic data) for over 600 compounds. Ion energetics data for over 16,000 compounds. Thermophysical property data for 33 fluids.
http://webbook.nist.gov/

Naval Research Laboratory Electronic Structure Calculations Database
Band structure, total energy, density of states, and derived quantities for most Periodic elements, hydrides, carbides, nitrides, and oxides.
http://cst-www.nrl.navy.mil/database.html

The NCI database
Database holding information on fundamental physical constants, atomic spectra, molecular spectroscopic data, ionization data, X-Ray and Gamma-Ray data, radiation dosimetry data, nuclear physics data, and condensed matter physics data.
ftp://helix.nih.gov/ncidata

Quantum Chemistry Literature DataBase
A literature database on ab initio MO calculations published in major journals of Chemistry, Physics and Computer Science since 1978.
http://qcldb2.ims.ac.jp/

Quest Consultants
Source of thermodynamic data useful in handling phase transitions. The data are given as a function of temperature and pressure for single components and mixtures. Also links to Bubble Points, Dew Points, and Enthalpy-Pressure Diagram Pages.
http://www.questconsult.com/~jrm/thermot.html

The Radiation Chemistry Data Center of the Notre Dame Radiation Laboratory
Chemical property data (such as reaction rate constants, and redox potentials,) historic bibliographic information from the RCDC bibliographic database, and recent papers in radiation chemistry and photochemistry. Also includes an on-line continuation of the Biweekly List of Papers in Radiation Chemistry and Photochemistry.
http://allen.rad.nd.edu/

Surface Tension Data
Surface tension values of some common test liquids for surface energy analysis.
http://www.surface-tension.de

University of Alberta
Tables of chemical data for Fundamental Physical Constants, Relative Atomic Masses of the Chemical Elements, Molar Thermodynamic Properties of Pure Substances, Molar Thermodynamic Properties of Aqueous Solutes, Ionization Constants of Aqueous Monoprotic Acids, Ionization Constants of Aqueous Polyprotic Acids, Solubility Products of Slightly Soluble Salts, Stability Constants of Aqueous Complex Ions, and Aqueous Standard Reduction Potentials.
http://www.chem.ualberta.ca/courses/plambeck/p101.new/p0040x.htm