Mimir

In Norse Mythology, Mimir was a wise being. Possibly the son of Bolthorn. In some myths a god and in others a giant. He was sent as a hostage by the Aesir to the Vanir. The vanir became mad and cut his head off. Odin preserved his head so he could seek wisdom from it and placed it next to Mimir's well.

Source: Nicole Cherry, A list of Norse Beings, originally at www.ugcs.caltech.edu/~cherryne.
Copy still available at www.angelfire.com/wy/wyrd/list.html