Woolen socks handmade with the naalbinding knitting technique. Naalbinding was the knitting technique used during the Viking Age and it has survived in the Nordic Countries till this day. If you want historically accurate Viking Age socks, then these are the ones to go with. They do an excellent job with both ventilating and keeping your feet warm - both with and without your historical shoes.
Clothing remains from the Viking Age are rare due to wool, linen and such not surviving very well. There is, however, one known naalbinding sock that has survived from the 900s. It is from Coppergate in York, England, that was a Viking settlement at the time.