DWSIM is a Brazilian open source CAPE-OPEN compliant chemical process simulator, available for download from here. It runs on Windows in native mode, and Linux and Mac OS X through Mono.
DWSIM allows chemical engineering students and chemical engineers to better understand the behavior of their chemical systems by using rigorous thermodynamic and unit operations' models with no cost at all. 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.