Ns is a discrete event simulator targeted at networking research. Ns provides substantial support for simulation of TCP, routing, and multicast protocols over wired and wireless (local and satellite) networks.

Network simulator designed from the outset for maximum speed and scalability, with real-time simulation as an achievable goal.

OMNeT++ is an open-source, component-based simulation package built on C++ foundations. It offers a C++ simulation class library and GUI support (graphical network editing, animation).

a modular network simulator featuring a java GUI

SSF (Scalable Simulation Framework) is a public-domain standard for discrete-event simulation of large, complex systems in Java and C++. Several SSF implementations and a large number of open-source protocol models and other components exist.

a C-based simulation language for sequential and parallel execution of discrete-event simulation models

Tool for generation of realistic internet topologies, with export to several network simulators (currently ns2, JavaSim, and SSFNet).

The x-kernel protocol framework can be used as a complete simulation environment for TCP/IP networks.