This unfortunately named product is actually the PPC platform's equivilent of VMWare. It allows you to run the Mac OS within a safe little hardware abstraction bubble that runs on top of linux. Thus it provides the handy feature of being able to run the Mac OS side by side with Linux on the same box at the same time, meaning you can use Linux for your sysadmin-like tasks, drop into Mac-on-linux for those two or three mac apps you just have to use, and still have your linux kernel, with its memory protection and all, still functional after the Mac OS inevitably crashes.

Mac-on-linux was created by some swedish company called Ibrium HB. However it is fully open-source and free and GPLed, and can be found bundled with most PPC linux distributions (LinuxPPC and Yellow Dog Linux i know of). In this respect it is maybe a tiny bit closer to a less abstract version of Plex86 (FreeMWare) than to VMWare.

Hardware abstraction is not emulation!

Since perdedor's post, things have been changing for Mac-On-Linux, and it is now directly competing with Mac OS X's Classic run-time environment. It sports much better 2d graphics acceleration at this time and complete sound support, contrary to what perdedor claims. It now has built in VNC support, G4 support, and 2.4 Kernel support, and most notably, interrupted session support. That is, it allows a person to hit F12 and watch as Mac-On-Linux immediately turns off, and pick up precisely where they left off upon Mac-On-Linux's next run. Very Cool. Also, plans for a regular interface to Linux's Network Device are in the works, which will allow for Mac-On-Linux behave like any other Linux program in respects to networking. Hopefully, Mac-On-Linux will keep progressing for a good while to come.