Is There A Cisco-IOS Emulator?

Peter Berghmans asks: "Emulators everywhere today. It's not very difficult to run several OS-es at the same time. Just to try out something new, to develop, to learn. But I can't find an emulator for the Cisco-IOS. Although it would be usefull to learn about the OS of one of the most important pieces of hardware on the Internet: routers. Is there some development to run IOS under Linux?"