DESQview
DESQview was a text mode multitasking program running on top of DOS. It was developed by Quarterdeck Office Systems. DESQview was released in July 1985, four months before Microsoft introduced the first version of Windows. DESQview was the successor to Quarterdeck's DESQ.
DESQview was not a full-fledged GUI operating system. It was a quasi-GUI shell that ran on top of DOS. DESQview allowed users to run multiple DOS programs concurrently in windows.