OS/2
Originally collaboratively developed by Microsoft and IBM to replace DOS, OS/2 version 1.0 (released in 1987) had no GUI at all. Version 1.1 (released 1988) included Presentation Manager (PM), which looked a lot like the later Windows 3.0 UI. After the split with Microsoft, IBM developed the Workplace Shell (WPS) for version 2.0 (released in 1992), a quite radical, object-oriented approach to GUIs. Microsoft later imitated some of this look in Windows 95.
Read more about this topic: History Of The Graphical User Interface, The 1990s: Mainstream Usage of The Desktop