GNS3- Graphical Cisco Router Emulator.

I found GNS3 is a very useful Cisco router and PIX firewall simulator. Please find the descriptions I copied from their website.

You can download the software from http://www.gns3.net

GNS3 is a graphical network simulator that allows you to design complex network topologies and to launch simulations on them.
To allow complete simulations, GNS3 is strongly linked with :

Dynamips, an IOS emulator which allows users to run IOS binary images from Cisco Systems.
Dynagen, a text-based front-end for Dynamips.
Pemu, a PIX emulator.
GNS3 is a excellent complementary tool to real labs for administrators of Cisco networks or people wanting to pass their CCNA, CCNP, CCIP or CCIE certifications.
It can also be used to experiment features of Cisco IOS or to check configurations that need to be deployed later on real routers. This project is an open source product that may be used on multiple platforms, including Windows, Linux, and MacOS X.
Features overview
Designing high quality complex network topologies.
Emulating Cisco routers and PIX firewalls.
Simulating simple Ethernet, ATM and Frame Relay switches.
Load and save in Dynagen's INI-like format.
Image export (JPEG, PNG, BMP and XPM).