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 don't think we'll see more simply because once Apple made the transition to Intel, it became somewhat "easy" to install an Intel supported version of OSX on vanilla hardware. I don't think it's the lawsuits keeping the emu guys at bay, I think it's just the "why bother" factor. If it's nostalgia, I completely understand.
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.
I heard to fight the Hackintosh crowd that Apple is considering migrating Mac OS X to ARM based devices for their next Macintosh evolution. That way they can control what devices run Mac OS X better, and since iOS is Mac OS X based and runs on ARM chips it would be easier to port Mac OS X to ARM based Macintosh systems and possibly have a virtual machine on ARM Mac OS X to run iOS apps as well.
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.
nit: in the manufacturing and business worlds, two years (let's give benefit of the doubt and presume to assume that "until 2014" is "until 31 Dec 2014") is not enough time to be leisurely. I do suspect, however, with their PA Semi purchase, that Apple is in a good position to know who to buy and how to buy them and it wouldn't be a strenuous search.
All assuming, of course, that Apple is interested in such a purchase.
This would be a sad move, but it is completely their call. It might open the door to emulators being back in style. vMac and Basilisk were my weapons of choice. And there was Shapeshifter/Sheepshaver on the Amiga... Good times.