Also an acronym for Mac On Linux, a very powerful MacOS emulator for linux. The catch, however, is that it really isn't an emulator. Through judicious kernel patching, it manages to send the instructions straight to the processor, making it very fast. Unfortunately, this means it only works on hardware that could run MacOS anyway.
It's currently under very active development. It has support for network devices, console video, sound, pretty much any sort of storage device, and is fairly stable. The only major flaw, when it does segfault, it tends to bring down lots of other stuff, possibly because of the kernel patching.
This is especially annoying when it brings down your sshd on your machine 600 kilometers away, with no remote reboot capabilities...