GNS3- Graphical Network Simulator - New Release
GNS3 is a excellent tool that uses dynamips to simulate routers running real Cisco IOS. You must have rights on your CCO account to download the IOS. It also includes the PIX emulator so you can check out your PIX/ASA configs.
Only piece missing is the switch simulators. You can combine GNS3 with VMware Workstation to build entire lab environments. I have one friend that has most of his voice lab for CCIE built using VMware and GNS3. Good stuff.
Runs in Windows/Linux/OSX.