For years, I've wanted a tool that would do for routers what VMWare does for computers. Sure, there are some really great simulators available such as the Sybex CCNA Virtual Lab (which I used to renew my CCNA), but a simulator is not the same as a router. A simulator is a great learning tool because of its structured labs, but it doesn't support the entire IOS command set and it doesn't allow you to connect to real or virtual PCs and networks.
Recently, I ran across Dynamips and Dynagen. These two open-source tools work together to allow you to virtualize routers in much the same way that VMWare, VirtualPC, and similar tools allow you to virtualize computers. Dynamips is the backend that does the actual emulation and Dynagen is the front-end that provides easy-to-use management tools for Dynamips. There is a GUI called GNS3, but I tend to prefer command-line configuration of Cisco devices. Windows users can download a complete package that includes Dynamips, Dynagen, WinPCap, sample labs, and a tutorial. Linux/UNIX users have several download options as well. Support is provided through tutorials and a forum. The tutorial is excellent and reasonably easy to follow. When running under Windows, WinPCap allows you to integrate the virtual router with physical networks and devices. I actually used my virtual router to perform classroom demos today in our Cisco router seminar while fully integrating with the classroom network.
There are some limitations: By default, the tool uses 100% of your CPU, but a configuration guide explains how to avoid that. The tool also seems to exhibit some instability when changing interface parameters, but that could be a result of my newness with it. It doesn't support the entire line of Cisco routers; just 7200s, 3700s, 3600s, and 2600s. Some documentation suggests that it also supports 1700s. I have also read forum postings by people who use it with PIX software images (One more thing to try!). You do have to provide your own IOS software image. All-in-all, I'm quite impressed with it...so much so that I wanted to share this information with you right away. Hope you find it helpful.