The Common Open Research Emulator (CORE) is a tool for emulating networks on one or more machines. You can connect these emulated networks to live networks. CORE consists of a GUI for drawing topologies of lightweight virtual machines, and Python modules for scripting network emulation.
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++.
Liowsn (Linux for works with WSN) was created to assist teachers, students and people involved with projects of Wireless Sensor Network (WSN). The operatingsystem is Ubuntu 9.10 remastered and it contains several tools for simulation and testing installed to work with WSN.
Discussion Group: http://groups.google.com/group/liowsn-users
User manual: https://dl.dropbox.com/u/4912938/LiowsnManual_EN.pdf (English Version)
https://dl.dropbox.com/u/4912938/LiowsnManual_PT.pdf (Portuguese Version)