OS/2

OS/2 was an operating system originally developed jointly by Microsoft and IBM. Between the release of Windows 3.0 and OS/2 1.3, Microsoft stopped all cooperation with IBM and IBM developed OS/2 on its own, starting with OS/2 2.0.

Microsoft later used OS/2 code to develop Windows NT 3.1. OS/2 code was later used by Serenity Systems to develop eComStation, which in turn was used by Arca Noae LLC to develop ArcaOS.

Warp era

 * OS/2 Warp 5
 * Version 5.0
 * Codename Gandalf or Merlin 2
 * Developed 2003, scheduled for release in 2005?
 * Development started but project was later scrapped.