MCX is a GPU-accelerated, general-purpose, physically-accurate and feature-rich 3-D light transport simulator. It is one of the fastest simulators because it can use tens of thousands of GPU threads to simulate photons in parallel.
CDNsim is a GNU/LINUX simulation tool for CDNs, written in C++ (core) and python (GUI wizard). It models: redirection policies, cache policies, TCP/IP, batch simulations, statistics extraction and more. CDNsim is uses the OMNet++ library
A perl/Gtk2 GUI for VNUML (http://www.dit.upm.es/vnuml/). This GUI will let you quick prototype your VNUML topology by graphically placing, linking and editing network elements, reading/writing VNUML files and run your simulation right from the GUI.