1986: GEOS - History of Desktop OSs
GEOS was an 8-bit operating system with a window- and icon-based GUI for the second generation of Commodore's C64 system. Developed by Berkeley Softworks, GEOS was the world's third most popular operating system behind DOS and Mac OS.
For affordable home computers, the operating system dramatically changed the user interface from a command line system and introduced a graphical user interface that was used with a mouse. Later released versions of GEOS also supported the Commodore 128 and came even in a (free) version to run as an alternative OS on the Apple II in 2003 (AppleGEOS) and is still offered today.