MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming
GNS3 - GNS3 is a graphical network simulator that allows simulation of complex networks.
ns-3 - a discrete-event network simulator for internet systems
Free SNMP Agent Simulator - Verax Systems provides applications covering the entire IT & Telco service lifecycle from definition through provisioning and monitoring up to billing.
Wolfram Mathematica - Mathematica has characterized the cutting edge in specialized processing—and gave the chief calculation environment to a large number of pioneers, instructors, understudies, and others around the globe.
Paessler Multi Server Simulator - Paessler Multi Server Simulator is inexpensive and robust software that helps you power massive-scale testing.