1a. Quarterdeck DESQview
Developed by Quarterdeck Office Systems, DESQView was a popular "Shell" for DOS which enabled a primitive form of multi-tasking of applications. This allowed popular productivity applications such as Lotus 1-2-3, WordPerfect and Harvard Graphics to be task-switched. In combination with a special memory manager known as QEMM on Intel 286, 386 and 486-baeed PCs, it allowed users to take advantage of more than 640K of memory and have several applications running in resident memory simultaneously.
DESQView, which was released in July 1985 -- only a few months before the release of Windows 1.0, was not the first task switcher for PC's -- that distinction goes to IBM's TopView, which never particularly caught on. DESQView itself would soon find itself in competition with Windows 386 and in 1990, Windows 3.0, which was a true, although non-preemptive multitasking GUI. By 1992, Quarterdeck did eventually release a full GUI version, of DESQView/X, but by then it was too late -- Microsoft had seized control of both the OS and GUI market for DOS with Windows 3.1 and its Office suite of native applications.