It STILL is in the very early alpha stage and may actually have been abandoned.
It won't run native OS X code, but you can compile code with Objective-C or other languages to run on GNUStep between Linux, Windows, and Mac OS X. It is based on NextStep, or OpenStep that Steve Jobs had at Next before Apple bought them out and turned it into Mac OS X.
Though that seems transparently different to me than this project.
http://pearpc.sourceforge.net/about.html
Mac OS X PowerPC emulation is spotty at best.
Mac On Linux? http://mac-on-linux.sourceforge.net/
Runs the PowerPC Mac OS X on PowerPC Linux systems.
Most that I know of aren't emulators but virtual machines for the PowerMac/CHRP platform that happen to run the PowerPC Mac OS X on them.
There would be more I am sure, and for Intel Macs, but Apple can just threaten lawsuits to discourage more of it. If the ability to run Mac OS X apps on other platforms exists, Apple stands to lose a lot of money on hardware sales. The Hackintosh market, for example uses cheap PC hardware to run Mac OS X on a non-Apple branded computer.
I did install OSX 10.2 Jaguar a couple of weeks ago on PearPC because my son is obsessed with the different releases that came out before he was born. I wanted to show him all that pin-stripey goodness.
They of course would have to buy a chip maker that can make 64 bit ARM CPUs with at least quad core to save on costs. I think Apple wants to get rid of buying chips from Samsung because of the iOS vs. Android lawsuits.
All assuming, of course, that Apple is interested in such a purchase.