Active Thermochemical Tables - This has the most accurate values of enthalpy data (frequently updated). Unfortunately,
this database is somewhat limited in scope.
NIST Chemistry WebBook - Lots of valuable data on thermochemistry, spectroscopy, mass spectrometry, etc.
See links to other NIST databases
JPL Data Evaluation #18 - Section 6 has enthalpy and entropy data for many gases of atmospheric interest.
Section 3 and Section 5 has a few equilibrium constants and a number of Henry's Law
Constants (!! but read the Introductions to these sections for notes on units).
