NIST atomic spectra database: http://www.nist.gov/pml/data/asd.cfm

NIST chemical kinetics database: http://kinetics.nist.gov/kinetics/index.jsp

Gas kinetic data: http://www.iupac-kinetic.ch.cam.ac.uk/

List of various thermodynamic web sites: http://tigger.uic.edu/~mansoori/Thermodynamic.Data.and.Property_html

Constants and Units: http://physics.nist.gov/cuu/Constants/index.html?/codata86.html