The New York Times has a nice article about the Escape key:
“It’s the ‘Hey, you! Listen to me’ key,” says Jack Dennerlein of the Harvard School of Public Health. According to Dennerlein, an expert on how humans interact with computers, the escape key helped drive the computer revolution of the 1970s and ’80s. “It says to the computer: ‘Stop what you’re doing. I need to take control.’ ” In other words, it reminds the machine that it has a human master. If the astronauts in “2001: A Space Odyssey” had an ESC key, Dennerlein points out, they could have stopped the rogue computer Hal in an instant.