The story of Leda and the Swan is one of the most popular in all of Greek myth. Leda was the daughter of King Thestius and Eurythemis, and she married Tyndareos, the king of Sparta. She was seduced by Zeus, and as with most of his conquests, Zeus took an alternate form during their encounter. In this case, it was the form of a swan.
The usual version of the story says that that night she layed two eggs, one the result of her encounter with Zeus, the other due to her husband Tyndareos. The first egg had Pollux and Helen (of Troy), and the second egg, of mortal parentage, contained Castor and Clytaemnestra (who would later marry Agamemnon).
Leda and the twins Castor and Pollux are well-known constellations, and the rape of Leda by Zeus is a common subject in art, with Leda's response varying from terror to bliss.