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.
ns-3 - a discrete-event network simulator for internet systems
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.
Paessler Multi Server Simulator - Paessler Multi Server Simulator is inexpensive and robust software that helps you power massive-scale testing.
Free SNMP Agent Simulator - Verax Systems provides applications covering the entire IT & Telco service lifecycle from definition through provisioning and monitoring up to billing.