I think dmenu is a good example of this. It's a common launcher application that's (at least) known to all i3 users. Here's a succinct explanation from the man page:
"dmenu is a dynamic menu for X, which reads a list of newline-separated items from stdin. When the user selects an item and presses Return, their choice is printed to stdout and dmenu terminates. Entering text will narrow the items to those matching the tokens in the input.
dmenu_run is a script used by dwm(1) which lists programs in the user's $PATH and runs the result in their $SHELL."
In practice you press Super + D, type a few letters to match the name of your program (such as "fox" for "firefox"), and press enter. It's so fast that any delay is near imperceptible even on older hardware. It also accepts command line arguments if you don't care to read the stdout from the process. This is a stupidly simple program that works with no configuration unless you want to change the font size.