Modelica

Interesting open source project, much large in Europe than the US:
Modelica and the Modelica Association
The object-oriented modeling language
Modelica is designed to allow convenient,
component-oriented modeling of complex physical systems, e.g., systems containing
mechanical, electrical, electronic, hydraulic, thermal, control, electric power
or process-oriented subcomponents. The free Modelica language, free Modelica
libraries and Modelica simulation tools are available,
ready-to-use and have been utilized in demanding industrial applications,
including hardware-in-the-loop simulations. The development
and promotion of Modelica is organized by the non-profit
Modelica Association.