CoolProp is an open-source, cross-platform, free property database based in C++ 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. Wrappers are currently available for EES , Python, Microsoft Excel, C#, Octave, MATLAB and a DLL for other windows applications. A nice alternative to costly REFPROP!
CoolProp can be downloaded free from its website.
Organic Rankine Cycle 