42

From George Will in yesterday's Washington Post:

Like many New Yorkers leaving home for work on April 15, 1947, he wore a suit, tie and camel-hair overcoat as he headed for the subway. To his wife he said, "Just in case you have trouble picking me out, I'll be wearing number 42."

No one had trouble spotting the black man in the Dodgers' white home uniform when he trotted out to play first base at Ebbets Field. Suddenly, only 399, not 400, major league players were white. Which is why 42 is the only number permanently retired by every team.

(Read on here)