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.
Yans is a extensible, mixed-level, Monte-Carlo Method based, discrete event network simulator. Yans is divided into interacting layers, namely: core, topology, nodes, monitors; and is driven by a config-file that describes the setup of the network.