GEM from Digital Research
Essentially, GEM brings to the PC and other computers a user environment
from the family of Macintosh and earlier software products that ran on
the Xerox Star.
Held up as the great non-IBM hope in the user interface arena (1985).
GEM is extremely small, roughly 100K, and it allows only one task to
run at a time.
GEM uses icons as well as text to get messages across.