XEvIE
The X extension that let a single program intercept, modify or discard every key and mouse event before it reached its destination.
The X Event Interception Extension came from Sun in the early 2000s, motivated by accessibility. Tools for users with motor impairments needed more than seeing events after the fact, which RECORD offered, or inventing new ones, which XTEST offered. They needed to stand in the way: turn two rapid keystrokes into one, or let an on-screen keyboard interpret clicks before any window saw them.
XEvIE did exactly that, and therefore also everything a keylogger dreams of. Only one client could use it at a time, few programs did, and the code was barely maintained. It was dropped from the X.Org server around 2009. Accessibility tools went through XInput instead and, eventually, through the compositor under Wayland.