Wikipedia
Dynamips is an emulator computer program that was written to emulate Cisco routers. It has been created by Christophe Fillot who started his work in August 2005. Dynamips runs on FreeBSD, Linux, Mac OS X or Windows and can emulate the hardware of the Cisco series routing platforms by directly booting an actual Cisco IOS software image into the emulator. Dynamips emulates Cisco platforms 1700, 2600, 2691, 3600, 3725, 3745, and 7200.
Although Dynamips original development has been stalled since version 0.2.8-RC2, released in October 2007, development continues through the efforts of the GNS3 project and its volunteers; Dynamips is now up to version 0.2.14-dev on Windows, Linux and OS X, and version 0.2.8-RC2 on Solaris. There are a few add-ons written for it. One of the most popular is Dynagen, which is a front-end add-on that allows the use of an INI configuration file to provision Dynamips emulator networks. Another popular add-on is GNS3, a graphical front end for Dynamips and Dynagen. The source code is licensed under the GNU GPL.