xgc
The X graphics demo — a panel with a button for every setting in a graphics context and a pane where you can see what they do.
An X11 GC has a couple of dozen attributes, and the man pages describe them with a precision that helps nobody picture the result. xgc turns them into controls: raster function (GXcopy, GXxor, GXinvert and the rest), line style, cap and join styles, fill style, plane mask, foreground and background. Then you pick an operation — points, lines, arcs, rectangles, text, CopyArea — and run it in the test pane.
It also timed the operations, which made it a crude benchmark for X servers before x11perf took that job seriously. Mostly it served as a teaching aid: watching an XOR line erase itself explains more than three pages of Xlib documentation. The interface is built from Athena widgets and looks like a control panel no designer was ever allowed near.