NOTE updated February 26, 2013: This post contains old information. I created a page that lists all the open-source network simulators listed in the post below. I will keep the page up-to-date. Please see the List of Open-Source Network Simulators page.
Clack Graphical Router
Clack is based on a project at Stanford University and is based on their VNS project. This system is a basic tool that simulates the basic operation of a network of routers. It appears to be used mostly to teach programming. It runs a graphical user interface in a browser window and the code is open-source. A live demo is available on the project web site.
CORE (Common Open Research Emulator)
This project continues the work started in the IMUNES project. It has a GUI interface and offers a VMWare image for trying it out without installing anything. It runs on the FreeBSD operating system but the research paper that announced the tool also suggests it might run on Linux because the authors refer to Linux virtualization tools in the paper.
MLN (Manage Large Networks)
MLN is a perl program used to manage a set of User-Mode-Linux instances or Xen instances, which are the basis for the switches and routers in a simulated network. The user creates a configuration file, which can specify a large complex network of virtual instances and configure each instance. This project was last updated in 2009 and it has some excellent documentation. Also the tool can manage instances on Amazon’s EC2 service.
This network simulator tool emulates a large Openflow network. It uses Linux namespaces to create virtual Openflow switches. The web site indicates that the tool can support thousands of virtual nodes on a single operating system. Since it is focused on Openflow, Mininet is not be useful for general routing simulation but it will be an interesting tool to investigate at a later time.