A typical peephole in a door, allowing the person to see who is outside the door without opening it.
Numerous devices exist to serve specific purposes related to how a door should (or should not) be used. See:

Door chain - A device to secure door opening
Door closer – Mechanical or electromagnetic device to close an open door (in the event of a fire)
Door opener - Automatic door opening device activated by motion sensors or pressure pads
Door damper – A hydraulic device employed to slow the door's closure Door knocker
Doorstop – used to prevent the door from opening too far or striking another object Espagnolette (for a window)
Fingerplate
Letter box or mail slot
Peephole