In Norse Mythology, Sif is the golden-haired wife of Thor and is thought to have originally been a fertility goddess who oversaw harvests. She is rarely mentioned in myths, except when in reference to Thor or "Sif's hair" as a euphemism for gold. Sometimes she is referred to as the mother of Ull even though his is not listed as one of Thor's children. One of Venus' volcanoes is named after the goddess, as most of the planet's features are named after mythological figures.
The one myth where she is prominent in is the story of Loki and her hair. Loki crept into her bedroom at night and chopped off all her hair for fun. When Thor heard of Loki's mischief, he threatened to break him into pieces unless he could replace her hair with a head of hair made of gold by the black elves. So, Loki went to the home of the sons of Ivaldi and begged them to spin fine golden hair for Sif and to cast a spell on it so it would grow like normal hair on her head. Dvalin agreed to weave the hair and Loki presented it to Sif.
Snorri Sturlason mentions Sif in his Skaldskaparmal when a giant named Hrungnir is going to drag her and Freya away to his home. Although both goddesses are remembered for their beauty, Freya is often the victim of giants and their pranks, Sif rarely is and some scholars believe they may be different manifestations of the same goddess although they are from different houses of gods.
Sif is also mentioned in the Poetic Edda when she presents Loki with mead in a crystal cup, attempting to calm him down while he taunts the other Gods over a meal in the following passage:
Then Sif came forward and poured mead for Loki in a crystal cup, and said:
"Hail too thee, Loki, | and take thou here
The crystal cup of old mead;
For me at least, | alone of the gods,
Blameless thou knowest to be."
Lokasenna
Sources:
http://www.pantheon.org/articles/s/sif.html
http://www.xenoscience.com/pixie/
http://www.windows.ucar.edu/tour/link=/mythology/sif.html
http://www.thorshof.org/sif.htm
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