xf86-input-evdev
The X server's generic driver for the Linux kernel's evdev interface, and the reason a USB mouse could be plugged in without restarting X.
For years the X server read mice and keyboards with drivers of its own, xf86-input-mouse and xf86-input-keyboard, and every device needed an InputDevice section in xorg.conf. Meanwhile the Linux kernel had grown a uniform interface, /dev/input/eventN, where every device delivers the same kind of events. The evdev driver read from there and handled almost anything with buttons or axes.
It became essential with hotplugging: X server 1.4 in 2007 could pick up new devices via HAL, and 1.8 in 2010 via udev, and evdev was what took care of them. Peter Hutterer at Red Hat maintained it for most of its life, and he was also the one who made it redundant by moving the logic into libinput.