GNS3 is a powerful network simulator that runs real Cisco software. Their main site can be found here.
Below you will find preconfigured GNS3 labs that can be downloaded and run on any machine using the new export feature introduced in version 1.5. Please note, these are intended to be used as framework topologies on which to build and configure – not necessarily complete networks.
Instructions for downloading and running these labs can be found on the Configuration Instructions page.