A number of materials were used to make ropes in the Viking Age. Not surprising then it was the materials at hand which were used. These included wool, horsehair, sealskin and lime bast. Lime bast? Well, this comes from the bark of the lime tree.
The derived rope is a one of the most frequently recovered rope remains found during excavations at Roskilde Fjord (no indications as to whether this in part may be because it deteriorates slower than other organic materials).
The Gislinge Boat Project at Roskilde Viking Ship Museum has posted a short video about the process of turning the bast/bark into rope. This is truly superb, and takes history and archeology to a practical level.