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.
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.
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).