DWSIM is an open source CAPE-OPEN compliant chemical process simulator for Windows and Linux. It is available for download from here.
DWSIM allows chemical engineering students and practising engineers to model process plants by using rigorous thermodynamic and unit operations' models. Since DWSIM is open source, they can see how the calculations are actually being done by inspecting the code behind during execution using free tools available elsewhere.
DWSIM's powerful Thermodynamics engine is also available as a standalone dynamic link library (DLL) that can be linked against proprietary applications.