Blender - Blender is the open source, cross platform suite of tools for 3D creation.
FlexSim - Simulation software to model, simulate, predict, and visualize systems in manufacturing, material handling, healthcare, warehousing, mining, etc.
FreeCAD - An open-source parametric 3D modeler
ns-3 - a discrete-event network simulator for internet systems
MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming
Paessler Multi Server Simulator - Paessler Multi Server Simulator is inexpensive and robust software that helps you power massive-scale testing.