From Larry Osterman's blog comes this little known fact about DOS 4.0, I wonder how different the operating landscape would have been if this came ever made it to market?
DOS 4 was a preemptive real-mode multitasking operating system for the 8086 family of processors. It had a boatload of cool features, including movable and discardable code segments, movable data segments (the Windows memory manager was a version of the DOS 4 memory manager). It had the ability to switch screens dynamically – it would capture the foreground screen contents, save it away and switch to a new window.