The worldwide famous QEMU software is now ported on MacOS, under the name of Q:
Run Windows, Linux and a lot more Systems on your Mac. Q is a feature packed cocoa port of QEMU: Switch fast between guest PCs. Save and restart guest PCs at any stage. Easily exchange Files between Host and Guest. Q makes use of OS X most advanced technologies like openGL and coreaudio to accelerate your experience with your guest PC.
More informations and the universal binary (running on x86 and PPC architectures) package are available here.