GEM (Graphical Environment Manager) began its life in 1985 as Digital Research’s graphical shell for CP/M. It made its way to MS-DOS and was the Atari ST’s interface. Caldera, which ended up owning GEM, open-sourced it in 1999. Since then, enthusiasts have updated and extended the code, which is typically released as FreeGEM or OpenGEM.
You can download an OpenGEM package that runs on just about any PC and includes apps–I recommend the “Full CD” package of FreeDOS, a DOS workalike that comes with OpenGEM. Just don’t expect too much: GEM retains its primitive look-and-feel so it can run on old hardware.