ns-3 - a discrete-event network simulator for internet systems
Cisco Packet Tracer - Download Cisco Packet Tracer, an innovative network simulation and visualization tool used by Cisco Networking Academy students and instructors.
Paessler Multi Server Simulator - Paessler Multi Server Simulator is inexpensive and robust software that helps you power massive-scale testing.
MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming
Mininet - Mininet is a robust and inexpensive Instant Virtual Network designed to function on standard PCs like Laptops and Desktops.
GNS3 - GNS3 is a graphical network simulator that allows simulation of complex networks.