JBotSim is a multipurpose dynamic network simulator that allows to quickly prototype a distributed algorithm and test its behavior on a set of static or mobile nodes. If mobile, the nodes can be controlled by the mouse or by program.
Simured is a multicomputer network simulation whith visual interface to see packet movement on the network. It is multi platform and there are versions in Java and C++.
Netsim is a mobile ad hoc network simulator targeted at large heterogeneous node configurations. It is written in Java and is easily extensible through its modular concept.