GEOS was a cool GUI
developed by Berkeley Softworks
I got GEOS for my C64 and Windows 3.0 for my PC about the same time. I was surprised when I noticed that GEOS was actually a lot better than Windows - it had drag and drop file moving, printing and deletion and that sort of things... even really crude multitasking that worked better than Windows multitasking on my machine =)
Of course, my 386SX was faster and more capacityful than my C64 because it had a hard disk, so I (reluctantly) declared Windows the winner.
Oh, and yes, GEOS lives. These days, it's being developed by Geoworks, the Geoworks Ensemble package was a Windows clone that also worked on low-end machines. Geoworks' GEOS lives at least in my Nokia 9110 Communicator. The Commodore GEOSes are still being developed by other people - one of the promising versions is called Wheels, with versions for Commodore 64 and 128.