(Also Gefjon, Gefinn, Gefjun)

"giver"
"gift"

In Norse Mythology, Gefion was a goddess of fertility and agriculture. A member of both the Vanir and the Aesir, she may have been a handmaiden of Frigg or possibly an alter-ego of Freyja, the goddess of love and procreation, who was often referred to as 'Gefn'. She was the protector of women who died while still unmarried, and all virgins went to Gefion's hall after death to serve the goddess. However, while she is frequently described as being a virgin herself, The Poetic Edda and various other sources tell of her having four sons by a giant and later marrying Skjold, one of the sons of Odin. She symbolized luck, fertility, virginity and growth.

Her myth concerns the acquisition of land, and is possibly emblematic of the Aesir's tendency to trick human beings. Gefion was sent to Midgard by Odin to find new lands, and she came upon Sweden, which was ruled by a king named Gylif. The goddess disguised herself as a beggar woman and entertained the ruler. In return, he offered her as much land as could be plowed by four oxen, working for one day and one night. She traveled to Jotunheim and found her four oxen sons, then went back to Midgard. Where Gefion and her sons plowed, a lake called Logur was formed, and then a great chunk of land was torn from Sweden and dragged into the sound. She named the land Zealand (though other islands were included as well) and it became part of Denmark, where she would eventually settle after her marriage to Skjold.

It is not known whether the entertainment that Gefion offered to Gylif was sexual or not, but some time later Loki accused the goddess of selling herself.

Bragi the Old wrote:

Gefion dragged with laughter
from Gylif liberal prince
What made Denmark larger,
so that beasts of draught
the oxen reeked with sweat;
four heads they had, eight eyes to boot
who went before broad island-pasture
ripped away as loot.

One of Copenhagen's largest monuments is the Gefion Fountain, created in 1908, which is located near the statue of Hans Christian Anderson’s Little Mermaid. The statue depicts four oxen pulling Gefion in a chariot, set atop a platform surrounded by stones that is supported by two levels of cascading water.

Sources:
http://www.pitt.edu/~dash/gefion.html
http://www.pantheon.org/articles/g/gefion.html