From the PearPC website[1]:
PearPC is an architecture-independent PowerPC platform emulator capable of running most PowerPC operating systems.
Released under the GPL, and written in C, C++ and assembler, PearPC runs on Linux, *BSD and Windows, and is known to run Mandrake for PPC, Darwin for PPC, and Mac OS X. Screenshots and a description of it running the latter are on OSNews[2]; this story made Slashdot[3]. It can use either a generic, portable emulated CPU, or a x86-only, JIT CPU that translates PPC instructions into x86 instructions on the fly. The authors claim CPU emulation speeds of 1/500 (for the former) and 1/40 (for the latter) of your real hardware. It is only onto version 0.1.1, so many things remain unimplemented, such as sound, LBA48 and AltiVec.
Installing OS X on PearPC is apparently contrary to its EULA, which states that it may only be installed on Apple-labelled or Apple-licenced hardware. In any case, using the Dock causes an infinite crash-loop, and I can find no mention of actually using the emulated system (not too surprising, given the speed...), so with any luck Apple won't set the lawyers loose just yet.
Other related projects include SoftPear, QEMU, SheepShaver and Basilisk.
(Disclaimer: I have not actually tried PearPC yet (I don't own OS X, and can't be bothered to download Mandrake or Darwin), so I can't speak from personal experience. I can't find an online copy of the OS X EULA; any kind person wishing to /msg me the appropriate lines of it would be smiled at. PearPC might well fry things ("Don't use it on important data, it WILL destroy them sooner or later!" - the developers), so use it at your own risk!)
[1] http://pearpc.sourceforge.net/
[2] http://www.osnews.com/story.php?news_id=7085
[3] http://apple.slashdot.org/article.pl?sid=04/05/18/2159257