A short science fiction/fantasy story by Philip Van Doren Stern. Notable primarily because it was the basis of the movie It's a Wonderful Life.
It was written in 1943 (It's a Wonderful Life was released in 1946), as a very short story, just 11 pages in my Pinnacle Books paperback edition. It wasn't well received; Stern couldn't get it published, and ended up printing 200 copies at his own expense. He gave these out as Christmas gifts for his friends, one of whom was a Hollywood agent. She showed it to others in the movie industry, and it ended up in the hands of Frank Capra, who made it into the aforementioned movie. This film didn't do to well either -- at least at first. Nowadays it is one of the best-known Christmas movies in the world.
I'm going to assume that you know the plot; if you don't, be warned, spoilers ahead. (I don't think that knowing the plot will spoil the story).
The story starts just before Christmas, with George Pratt standing on a bridge, contemplating suicide. He's a lowly bank teller, and feels that life has passed him by. Just as he is leaning over the railing, a nondescript old man warns him not to jump. (We never find out who the man is, although he's clearly magical). In the following conversation George says that he wishes that he had never been born, and the old man grants that wish. He doesn't feel any different, but he finds that he is now in an alternate time-line (my term, not Stern's) in which he never existed. This time-line has three primary differences:
- The man who had gotten the bank teller job in place of George embezzled $50,000, and then disappeared. This caused the bank to crash, with disastrous effects on the local economy.
- His wife had married another man, who turned to drink after the bank crash. It is currently a very unhappy marriage.
- His brother, who he had originally saved from drowning at a young age, ended up drowning, leaving his parents childless.
After seeing how awfully the world was with out his positive (if admittedly mundane) influence, George returns to the old man and asks for his old life back.
The two most notable differences between this story and the movie are the length (obviously), and George's life. Despite being tremendously shorter than the movie, the story is actually just about as good (not that I'm a big fan of either). All the stuff the movie adds about George's younger days is redundant, and the movie's sub-plot about the stolen money wasn't all that great anyway.
In the movie, George was clearly a 'good' person; he was constantly making sacrifices for others, and any differences he made in the world were supposed to be seen as an offshoot of his overall niceness. In the original short story, George is just a low-level, everyday worker -- the only special thing he did was save his brother from drowning, and no-one thought much of that at the time. It changes the moral somewhat; the movie's moral comes across as "good things come to good people", the story's simply as "everybody makes a difference".
You can find the original story in a number of collections; I don't have a complete list, but you might look for:
No, But I Saw the Movie edited by David Wheeler (Penguin, 1989)
Christmas Stars edited by David G. Hartwell (Tor, 1992)
The Other Side of the Clock: Stories Out of Time, Out of Place by Philip Van Doren Stern (Van Nostrand Rinehold, 1969; Pinnacle 1971)
There is also a novel by Stern, published in 1996, called The Greatest Gift: A Christmas Tale. I have not read it, but I guess that after the movie made it big, Stern decided cash in on its success by rewriting his story. (Stern is not making any money off of the movie itself; the copyright has expired).