SheepShaver provides an Open Source PowerPC Mac OS runtime environment. It allows you to run PowerPC Classic Mac OS software even if your operating system is different. To use this program, you will need a copy Mac OS and a PowerMac Rom image. SheepShaver can be distributed under the GNU General Public License (GPL).
Applications will run natively on a PowerPC-based computer. SheepShaver offers the first PowerPC G4 emulator without MMU to allow the execution of Mac OS Classic on other systems. The current CPU emulator uses basic JIT translation techniques to achieve speeds that are roughly one-eighth of native speeds.
These platforms are supported currently: Linux (i386, ppc and x86_64), MacOS X(i386, ppc), Darwin (ppc), FreeBSD 5.3, NetBSD 2.0, FreeBSD 5.3, Windows for x86. Note that I am not a Windows developer and I am very careless about the Windows version. You are welcome to join the effort!
Version 2.5 (Build from March 2021):