Where to find data for Gibbs energy, enthalpy, and entropy? Where can you find the enthalpy and Gibbs free energy of formation and entropy of compounds either as a data file or web API? Technically, I only need two since I can figure out the other one from two of them.
NIST webbook does have a lot of data, though they are not in any kind of an API form as far as I know.

http://webbook.nist.gov/chemistry/