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.
Cisco VIRL - Cisco VIRL is a next-gen network virtualization and orchestration platform that enables you to accomplish multiple tasks like creating and customizing simulated networks with third-party and Cisco objects by means of a graphical utility.