REAL  
   a modular network simulator featuring a java GUI
   http://www.cs.cornell.edu/skeshav/real/overview.html

Parsec  
   a C-based simulation language for sequential and parallel execution of discrete-event simulation models
   http://may.cs.ucla.edu/projects/parsec/

x-sim  
   The x-kernel protocol framework can be used as a complete simulation environment for TCP/IP networks.
   http://www.cs.arizona.edu/xkernel/

NIST ATM/HFC Network Simulator  
   
   http://www.hsnt.nist.gov/misc/hsnt/prd_atm-sim.html

SSFNet  
   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.
   http://www.ssfnet.org/

OMNeT++  
   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).
   http://www.hit.bme.hu/phd/vargaa/omnetpp.htm

ns2 network simulator  
   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.
   http://www.isi.edu/nsnam/ns/

Scalable Networks  
   Network simulator designed from the outset for maximum speed and scalability, with real-time simulation as an achievable goal.
   http://www.scalable-networks.com

BRITE  
   Tool for generation of realistic internet topologies, with export to several network simulators (currently ns2, JavaSim, and SSFNet).
   http://www.cs.bu.edu/brite/

Akaroa  
   Akaroa is a package for supporting the Multiple Replications In Parallel (MRIP) simulation technique to harness the computing power of a network of inexpensive workstations. Integration exists with the Ptolemy Classic, ns2 and OMNeT++ simulators.
   http://www.cosc.canterbury.ac.nz/research/RG/net_sim/simulation_group/akaroa/

J-Sim  
   A component-based, compositional simulation environment.
   http://www.j-sim.org/

cnet  
   A network simulator with a GUI and targeted at undergraduate teaching. Enables experimentation with various data-link layer, network layer, routing and transport layer networking protocols.
   http://www.csse.uwa.edu.au/pls/cnet/