Blinkenlights

From the overview:

Computers once had operator panels that provided an intimate overview of the machine’s internal state at any given moment. The blinking lights would communicate the personality of each piece of software. Since our minds are great at spotting patterns, developers would intuitively understand based on which way the LEDs were flashing, if a program was sorting data, collating, caught in an infinite loop, etc. This is an aspect of the computing experience that modern machines haven’t done a good job at recreating, until now.