Shadrach, Meshach and Abednego - who is understood as the fourth person in the fire?

Shadrach, Meshach and Abednego, who is understood as the fourth person in the fire? In the third chapter of book of Daniel after King Nebicanesor throws Shadrach, Meshach and Abednego into the furnace he sees four people in the furnace instead of three, who are talking with each other. - What is understand of who this person is? An angel, a [Christophany](http://en.wikipedia.org/wiki/Christophany) ? - What is the current Christian understanding of this? - How would a Jewish person in ancient times have understood this? - Is there any extra biblical Jewish sources/tradition naming the fourth person (such as a specific angel, etc)?