If you are a developer, you might know about VirtualBox. Mach, dyld, launchd - everything you'd expect.
I could not believe it - Linux is the leading powerful operation system, and nowadays there is NO ONE working emulator for the greatest computer of the past - Apple][.
While VirtualBox is an excellent tool, there is another option: QEMU, which is an open source machine emulator and virtualizer available under the GPLv2 license.