The Mac OS X Developer Previews include the development of the Classic Environment. In fact, you can load a built-in Mac OS 9 virtual machine!
SheepShaver is an Open Source PowerPC Mac OS run-time environment. That is, it enables you to run PowerPC Classic Mac OS software on your computer, even if you are using a different operating system. However, you still need a copy of Mac OS and a PowerMac ROM image to use this program. SheepShaver is distributed under the terms of the GNU General Public License (GPL).
Features:. SheepShaver runs macOS 10.7 through macOS 10.14. PowerPC G4 emulation on non-PowerPC platforms, direct execution otherwise. Basic but portable JIT engine (x86, x8664, mips). Copy and paste of text What's New in SheepShaver. SheepShaver is an Open Source PowerPC Mac OS run-time environment. That is, it enables you to run PowerPC Classic Mac OS software on your computer, even if you are using a different operating system.
However, you still need a copy of Mac OS and a PowerMac ROM image to use this program. SheepShaver is distributed under the terms of the GNU General Public License (GPL).