270? - December 6, 343: Age 73?
Yes! It's true! Santa Claus died on this day!
Nicholas' parents were were affluent Christians in what is now Turkey in a Greek city, Patara. Nicholas himself was an extremely pious infant. Legend has it that he carefully observed canonical feasts by abstaining from his mother's breasts on the appropriate days.
His parents died when he was quite young, and he devoted his inheritance to charitable works. The most famous story has it that he heard of an affluent man who had lost all of his money, including the dowry for his three daughters. The daughters had no prospects of marriage, and were facing life working in a brothel if something didn't change in the family fortunes. When Nicholas heard of this, he took a bag of gold, snuck up to the man's house at night, and threw it in the window. The man used the gold as a dowry, and married off his eldest daughter. Later, Nicholas returned and threw a second bag of gold into the house. Finally, with the third daughter, the father secretly kept watch, caught Nicholas in the act, showered him with thanks, and told the world what a great guy he was. This act of secret giving, combined with the proximity of his Saint-Day to Christmas has, through a complex series of transformations, given rise to the modern-day stories of Santa Claus as a bringer of gifts on Christmas Eve.
For his help to the poor, Nicholas became the patron saint of pawnbrokers. These three bags of gold morphed into the three golden balls that you see hanging outside pawnbrokers' shops in pre-literate days. He is also the saint of sailors and all things nautical, the patron saint of Greece, and the patron of children and students.
This latter association is probably because of another story: during a famine, an innkeeper caught and killed three children, pickling them in a tub of brine with the intention of selling them as ham. Nicholas found the three children and brought them back to life: this is seen in some of his statues showing three infants in a tub at his feet. Another version of the story has the three victims as adults, possibly the origin of the English legend of Sweeney Todd, the barber who murdered his customers and gave them to his girlfriend to bake into pies for her shop. Anyway, these three lucky people, whatever age they were, were revived by St. Nicholas.
Nicholas became the Bishop of the city of Myra. He was one of the participants in the First Council of Nicaea, a pivotal meeting of the Christian world that resulted in the first uniform Christian doctrine, called the Nicene Creed. The was a great controversy at the time about whether Christ was of the same substance as his Father, or whether he was simply a creation of his Father. Arius, a priest from Alexandria, espoused the latter view, which was voted down, and perhaps Saint Nicholas' contribution to the argument had an influence on that decision: he became so enraged by Arius' views that he rushed over to him and boxed his ears. (I had to look that up: it usually means striking someone on both ears at the same time. I bet it hurts.)
Nicholas' death is unusual in two respects. One, he died an old man in his bed: very unusual for a saint. Most of the early saints died in an untimely way in horrible torment. Two, his body stayed in one piece, we think. Getting the knuckle of a major saint could turn a small cathedral into a place of pilgrimage, bringing in tourists and boosting local trade. Thus most saints' bodies did not stay together, but were distributed/traded/sold for maximum merit/influence/profit. Originally his body was kept at Myra, but when the city was overtaken by Muslims some enterprising Italians "liberated" the remains, taking them to Bari, in Apulia, Italy. They have stayed there every since, oozing an oily substance known as "Manna di S. Nicola". They were examined by scientists in 1950 who concluded that Nicholas was a short man, only about 5 feet tall, and had a broken nose. Maybe Arius hit him back.
If you're wondering how the name "St. Nicholas" morphed into "Santa Claus", it was through the Germanic form of "Nicholas", which is "Klaus" (dropping the first syllable and eliding the second and third). The Dutch tradition of "Sinterklaas", based on St. Nicholas, came to North American through New Amsterdam (now New York) in the 17th century. The gift-giving, child-oriented stories of St. Nicholas are common to all Christian cultures, so the tradition was easily adopted by Christian people as they arrived in the United States.
Sources: Wikipedia, Catholic Online