Recently I've come across a comment online which talked about the materials used by Viking era shipwrights. Specifically about the materials used to join the hull's planks. It said that although iron rivets were indeed a popular material, they were not nearly as popular as wooden pins (I don't really know how to call it, since the comment wasn't originally written in English, but you get the idea - a long, wooden bolt). So, I went online searching and found a lot of conflicting information on that topic. From what I've gathered, the comment was wrong and the more popular material was indeed iron, which you can, for example see on the Oseberg wreck. But still, I have mixed feelings about it and would like a second opinion from someone that knows more about that topic.
Wooden pins were used in ship construction to help separate the deck from the hull and to provide other kinds of vertical support, but to the best of my knowledge, they were never used to join two pieces of wood together.
In the examples we have, iron rivets were used were the most likely joiners to survive. In many cases, the wood of Viking-Age ships or ship parts that have been buried completely deteriorate, leaving only rivets in the outline of a hull. A small boat could be made from about 50 rivets, but to take the recently excavated Salme burials in Estonia from the early 700s, thousands of rivets were sometimes used. At the opposite end of the Viking-Age world, iron working—perhaps to repair ship rivets—helped prove that a population from the Norse diaspora had in fact settled at L'Anse aux Meadows in Canada.
As a fun bonus, I think it's useful to know that the spaces between the overlapping planks were sealed with wool felt and yarn. It seems like this partially-worked wool was traded at least regionally as a commodity good, so archaeologists are sometimes able to identify ships made from wood from one place and caulked with wool from another.
The most authoritative publications are by Ole Crumlin-Pedersen, Michael Müller-Wille, and Jan Bill. Some of these are included among the excellent publication series of the Viking Ship Museum in Denmark. Of current staff publications, I'd point especially to Viking-Age War Fleets by Morten Ravn.
Edited to reflect the excellent comments by /u/Platypuskeeper below.