For PPC Mac emulation specifically, QEMU has been making strides in the past couple of years. It can competently run several versions of Classic Mac OS as well

For PPC Mac emulation specifically, QEMU has been making strides in the past couple of years. It can competently run several versions of Classic Mac OS as well as OS X up through 10.4. 3D acceleration is still missing, but last I checked was in the works.