TopView
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TopView was a text-mode MS-DOS multitasker written by IBM and released in 1985. In order to compete with various other graphical environments, IBM announced about TopView before it was finished, around the time they shipped their new PC AT computer (in 1984).
IBM considered that in order to harness the great power and memory capacity of the new machine (Intel 80286 with a 6 MHz clock and, in version 02, with 512 KB of RAM), some multitasking environment was necessary. TopView allowed IBM to sweeten the deal with customers, who were surprised that the new and very expensive machine did not come with an operating system able to use the hardware multitasking and protected-mode features of the new CPU, and was still running in 8088 emulation mode. However, TopView proved unnecessary to sell the new machines, and was quickly EOL-ed.
TopView ran in real mode on any x86 processor and could run well-behaved MS-DOS programs in windows. It did not make use of any later introduced virtualization features in the Intel 80386 processors.
TopView first introduced PIF files, or Program Information Files, which defined how a given MS-DOS program should be run in a multi-tasking environment, notably to avoid giving it unnecessary resources which could remain available to other programs. TopView's PIF files have been inherited by Microsoft Windows and allegedly by DESQview.
IBM promised to give TopView a user-friendly GUI, but never did.
TopView required MS-DOS 2.x and did not run with 3.0 and later releases any more.