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.

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.

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.