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++.
Pamvotis is a Wireless LAN Simulator for all the current physical layer extentions of the IEEE 802.11 (IEEE 802.11 a/b/g) standard and for the IEEE 802.11e Draft for Quality of Service in WLANs.
Sinalgo is a simulation framework for testing and validating network algorithms. It allows for quick prototyping and verification in freely customizable network settings. It is mainly suited for packet-oriented wireless networks.