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

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

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

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

NetSim - NetSim software was created to simulate various Cisco systems. Cisco software is known around the world as being one of the best network, routing and switch systems for businesses and private use. Read more about NetSim.

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