ec2-18-119-133-228.us-east-2.compute.amazonaws.com | ToothyWiki | RecentChanges | Login | Webcomic OperatingSystem? for the Amiga. Went through several incarnations, namely 1.3, 2.0x and 3.0x IIRC. One of the earliest OSs to display multitasking, a long time before Microsoft managed it.
To be picky, AmigaDOS? was the OS, WorkBench was the GUI built on top of it, in the same way as MSDOS and Windows3.1 .
WorkBench was OK as a graphical OS. Then MUI, "Magic User Interface" came along, and (if your Amiga was sufficiently powerful) made it very nice indeed.