twm
The window manager that shipped as X11's default, and so the first graphical desktop many Unix users ever laid eyes on.
Tom LaStrange wrote twm in 1987; the name started as "Tom's Window Manager" and was later rebranded "Tab Window Manager". From X11R4 in 1989 it was part of MIT's reference distribution, which gave it a reach no design could have earned on its own. It had flat title bars, an icon manager and menus configured in a plain-text .twmrc.
twm was an early adopter of reparenting — drawing its own frame around client windows. vtwm, tvtwm and later fvwm all began as patches to the same code. It still ships with X.Org and appears as the fallback when a desktop environment fails to start, which for most people is the only time they ever meet it.