Answer:
For the most part, very little. Iron was expensive. The common image of a viking decked out in armor with a huge sword and axe is quite wrong. If they had a metal helmet they were lucky. Usually they just wore furs and leather, armed with a spear (deadly weapon that uses very little Iron), and a wooden shield.
The types of metal armor that -was- available at the time included studded leather, ringmail and chianmail. Only princes were able to afford those, though.