William Sydney Who?

Yes, you have heard of him.

O. Henry was the pen name of American writer William Sydney Porter whose short stories are known for their clever twist endings.

Among his most famous stories are:
* "The Gift of the Magi" about a young couple who are short of money but desperately want to buy each other Christmas gifts. Unbeknownst to Jim, Della sells her most valuable possession, her beautiful hair, in order to buy a platinum fob chain for Jim's watch; while unbeknownst to Della, Jim sells his own most valuable possession, his watch, to buy jeweled combs for Della's hair. The essential premise of this story has been copied, re-worked, parodied, and otherwise re-told countless times in the century since it was written.
* "The Ransom of Red Chief", in which two men kidnap a boy of ten. The boy turns out to be so bratty and obnoxious that the desperate men ultimately pay the boy's father $250 to take him back.
* "The Cop and the Anthem" about a New York City hobo named Soapy, who sets out to get arrested so he can avoid sleeping in the cold winter as a guest of the city jail. Despite efforts at petty theft, vandalism, disorderly conduct, and "mashing" with a young prostitute, Soapy fails to draw the attention of the police. Disconsolate, he pauses in front of a church, where an organ anthem inspires him to clean up his life—whereupon he is promptly charged for loitering and sentenced to three months in prison, exactly what he originally set to do.
* "After Twenty Years", set on a dark street in New York, focuses on a man named "Silky" Bob who is fulfilling an appointment made 20 years ago to meet his friend Jimmy at a restaurant. A beat cop questions him about what he is doing there. Bob explains, and the policeman leaves. Later, a second policeman comes up and arrests Bob. He gives Bob a note, in which the first policeman explains that he was Jimmy, come to meet Bob, but he recognized Bob as a wanted man. Unwilling to arrest his old friend, he went off to get another officer to make the arrest.

More on Wikipedia.