OS/2

OS/2 (an IBM-ism for "Operating System/2") is an operating system developed by IBM and Microsoft. It was marketed as a "professional" upgrade from DOS and 16-bit Windows, and was the precursor of Windows NT, leading directly to modern Windows. For a brief history of OS/2, read its Wikipedia entry.

Although IBM discontinued OS/2 at the end of 2006, a third-party version called eComStation was available for several years from Serenity Systems, and now there is another third-party version available called ArcaOS that is still undergoing active development.