Wolfram SystemModeler is described as 'Developed by Wolfram MathCore, is a platform for engineering as well as life-science modeling and simulation based on the Modelica language' and is an app. There are more than 10 alternatives to Wolfram SystemModeler for a variety of platforms, including Windows, Linux, Mac, Web-based and Chrome OS apps. The best Wolfram SystemModeler alternative is Simulink. It's not free, so if you're looking for a free alternative, you could try Xcos or OpenModelica. Other great apps like Wolfram SystemModeler are SimulationX, Valispace, JModelica and VisSim.