Microsoft wasn't the only (or the first) company to begin extensive research and development of graphical user interfaces in the early 80s. Xerox pioneered the basic principles and others soon followed. Here are some contemporaries and competitors of Windows 1.0, I chose to primarily look at how they compare to Windows 1.0.

Amiga Workbench (AmigaOS)

Apple Lisa and Macintosh

Digital Research GEM

IBM TopView

Quarterdeck DESQ and DESQview

Tandy Deskmate

VisiCorp Visi On