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JavaSim
JavaSim is a component-based, compositional simulation environment.
http://www.javasim.org/
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
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/