Several tech companies began extensive research and development of graphical user interfaces in the early 80s after Xerox pioneered the basic principles. Here are some other early GUIs besides Windows that left an impact on the industry.

On the PC and compatibles:

VisiCorp Visi On

Digital Research GEM

IBM TopView

Quaterdeck DESQview

On other platforms:

Amiga Workbench (AmigaOS)

Apple Macintosh

Berkeley Softworks GEOS