Scilab is recommended for engineers, scientists, and educators who require a powerful computational tool without the associated costs of commercial software. It is also suitable for students and researchers who are looking to perform complex mathematical modeling and simulations.
Paessler Multi Server Simulator - Paessler Multi Server Simulator is inexpensive and robust software that helps you power massive-scale testing.
MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming
FlexSim - Simulation software to model, simulate, predict, and visualize systems in manufacturing, material handling, healthcare, warehousing, mining, etc.
Wolfram Mathematica - Mathematica has characterized the cutting edge in specialized processing—and gave the chief calculation environment to a large number of pioneers, instructors, understudies, and others around the globe.
ns-3 - a discrete-event network simulator for internet systems
GNU Octave - GNU Octave is a programming language for scientific computing.