SimulationX is described as 'interdisciplinary, multi-domain simulation software for the design, analysnd optimization of complex systems on a single platform' and is an app in the Education & Reference category. There are more than 25 alternatives to SimulationX for a variety of platforms, including Windows, Linux, Mac, Online / Web-based and Android. The best alternative is MATLAB. It's not free, so if you're looking for a free alternative, you could try Circuit Simulator or SciPy & Numpy. Other great apps like SimulationX are Spyder (Free, Open Source), Xcos (Free, Open Source), SymPy (Free, Open Source) and OpenModelica (Free, Open Source).
MATLAB is a numerical computing environment and programming language. Maintained by The MathWorks, MATLAB allows easy matrix manipulation, plotting of functions and data, implementation of algorithms, creation of user interfaces, and interfacing with programs in other languages.
Spyder (previously known as Pydee) is a free open-source Python development environment providing MATLAB-like features in a simple and light-weighted software, available for Windows XP/Vista/7, GNU/Linux and Mac OS X.
SymPy is a Python library for symbolic computation. The stated goals of the library are to become a full-featured computer algebra system and to keep a simple code base to promote extensibility and comprehensibility.