ns-3 - a discrete-event network simulator for internet systems

Paessler Multi Server Simulator - Paessler Multi Server Simulator is inexpensive and robust software that helps you power massive-scale testing.

FlexSim - Simulation software to model, simulate, predict, and visualize systems in manufacturing, material handling, healthcare, warehousing, mining, etc.

GNS3 - GNS3 is a graphical network simulator that allows simulation of complex networks.

Mininet - Mininet is a robust and inexpensive Instant Virtual Network designed to function on standard PCs like Laptops and Desktops.

VNX - VNX is a simple, fast, and highly effective virtualization tool developed to assist you in creating virtual network testbeds automatically, resulting in time-saving.