Qemu is a versatile open source emulator capable of emulating various guest systems on various host systems. This description will only cover Qemu's capabilities with respect to the emulation of Mac OS and OSX for the Apple PowerPC. By running Qemu on Linux, OSX, or Windows, users can currently emulate PPC Mac OS 9 up to PPC Mac OSX 10.4 (including the classic environments in PPC OSX). Community-provided Qemu builds for OSX and Windows are available through our forum. These include builds from both the official source and experimental builds.
Work-in-progress setup guides for the Mac OS X and Windows builds of Qemu are available.