In 1926, 11-year-old Johnny Sylvester was injured after being kicked in the head by a horse. Yankees star Babe Ruth sent Johnny a ball, on which he wrote, "I'll knock a homer for you."
That ball is up for auction, and could go for more than half a million dollars. Ruth's 1923 World Series pocket watch is headed to auction later this month.
UPDATE: The ball was sold on Thursday night for $250,641. The sale was timed to the 119th anniversary of Ruth's birth.