Athena Widgets (Xaw)
X11's free bundled widget set: flat, spartan, and equipped with a scrollbar that has confused people since the 1980s.
Xaw was built at MIT's Project Athena on top of Xt and shipped with X from the beginning. The set was small — buttons, labels, text fields, menus and the notorious scrollbar, where the left button scrolls down, the right scrolls up and the middle jumps straight to a position. It's still the one you meet in xterm, xcalc and xedit.
Because Motif cost licence money, Xaw became the default for free software that needed a GUI without asking anyone's permission. The Xaw3d variant later added bevelled edges as a drop-in replacement for the original library, giving the same programs a slightly less 1985 facade without recompiling.