CoolProp is an open-source, free property database that includes pure fluids, pseudo-pure fluids, and humid air properties. It is designed to be trivially simple to use from the Python programming language, and possible to use from other languages as well.
I. Krüger, A. Mehlhase, und G. Schmitz. Proceedings of the 9th International Modelica Conference, September 3-5, 2012, Munich, Germany, Linköping University Electronic Press, (November 2012)