GNS3 is a powerful network simulator that runs real Cisco software. Their main site can be found here.
In this section 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.
Labs are downloaded as zip files and can then be expanded to gns3project files. Full instructions can be found on the Setup page.