MS-DOS Executive
The shell of Windows 1.0 and 2.x: a file listing in a window, where you started programs by double-clicking their .EXE files.
No desktop, no icons, no program groups. Booting Windows 1.0 in 1985 got you a row of drive letters along the top and an alphabetical list of the files in the current directory beneath it. The menus held what you would otherwise have typed at the DOS prompt: copy, rename, delete, format a floppy. Programs launched when you double-clicked the filename, and documents opened only if their extension happened to be one Windows knew about.
It was essentially DOS with a mouse, which was rather the point: Microsoft was selling Windows to people who already knew DOS. Windows 3.0 replaced it in 1990 with Program Manager for launching things and File Manager for shuffling files. That split lasted five years, until Explorer in Windows 95 merged them again.