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.
Cisco Packet Tracer - Download Cisco Packet Tracer, an innovative network simulation and visualization tool used by Cisco Networking Academy students and instructors.
FlexSim - Simulation software to model, simulate, predict, and visualize systems in manufacturing, material handling, healthcare, warehousing, mining, etc.
Free SNMP Agent Simulator - Verax Systems provides applications covering the entire IT & Telco service lifecycle from definition through provisioning and monitoring up to billing.
Cloonix - Cloonix is an open-source virtual network creation framework based on KVM licensed under the AGPLv3 license.