Here’s an overview of some useful links and references found on the internet;

Netlib is a collection of mathematical software, papers, and databases
Freesteam is an open source implementation of IAPWS-IF97 steam tables in C
OllinTS is a thermodinamical server (also the Sim42 sources can be found here)
DWSIM – An Open Source Process Simulator written in VB.NET and C#