xautolock
A small program that locks nothing itself; it watches how long keyboard and mouse have been idle and launches a screen locker when time runs out.
Stefan De Troch and Michel Eyckmans wrote it in the early 1990s, when xlock was the locker everyone had and it never started on its own. The defaults are ten minutes and xlock, but -locker accepts any command, and -notify gives a warning a few seconds beforehand. With -corners, parking the pointer in a screen corner can mean "lock now" or "never lock" — the latter evidently invented by someone who gave presentations.
Early versions measured idleness by selecting events on every window themselves, which was both clumsy and leaky. Later builds use MIT-SCREEN-SAVER when the server offers it. The split between the thing that measures and the thing that locks survives in minimalist desktops, where xautolock and slock are still paired.