Zampone remained a local specialty until the advent of more intensive pig farming in the late 1800s, when people realized that it goes very well with the lentils almost all Italians eat to greet the New Year, at which point it rapidly became popular throughout the Peninsula.
There are two kinds of zampone: Raw and precooked, and though most Italians buy the precooked kind, which comes in a foil packet one gently boils for 20 minutes, the raw ones are much tastier. A raw zampone does take more work, however: Soak it overnight in cold water to soften the skin, wrap it in gauze, and simmer it for 4 hours in water to cover in a fish pot. Come serving time, remove it from the water, slice it into half-inch rounds, and eat it at once with lentils because it's not good cold (nor does it reheat well).
When eating a zampone one generally eats everything including the rind, which takes on a delightful gelatinous consistency. There are, however, people who find this gelatinous consistency abhorrent, and if you fall into this category there is also the cotechino, a 3-inch (8 cm) thick, 9-inch long sausage made with the same stuffing used for the zampone. The cooking time is about the same, and for those who would rather not watch a pot for hours there are precooked versions.
Cotechini and zamponi are not limited to New Year's; both are popular throughout the winter in Northern Italy, especially during cold snaps. They play an important role in bollito misto, a boiled dinner consisting of boiled meats and vegetables (the more variety the better) served with sauces that vary from place to place, though one can usually expect salsa verde and mostarda di frutta, among other things. They can also stand alone; see, for example, cotechino fasciato.
A general overview of salumi, Italian cold cuts.
Affettati Misti, a mixed cold cut antipasto