The most common substitute for pancetta is bacon, though it should be boiled to be a more accurate match. Pancetta is essentially Italian bacon that comes from the stomach of the pig, much like American varieties. Unlike American bacon, however, which is both cured and then smoked, the Italian type is only cured and so it lacks the common smoky flavor found in bacon. One way to get around this problem is to boil or blanch the bacon for a few minutes before using it as a substitute for pancetta in a recipe, as the water draws out some of the smokiness.
In many ways, American and Italian bacon are quite similar, and so American types are the most common substitute for pancetta in many recipes. It is important to keep in mind that they are cured and seasoned differently, however, so their flavors will not match perfectly. In a dish in which the pork is a major ingredient, such as a recipe that uses the meat as the primary source of flavor, Italian versions should be used if at all possible. Recipes that use it only as a secondary ingredient, however, often work quite well even when using a substitute for pancetta.
Bacon is the most common substitute for pancetta, since it is essentially the same cut of meat and is often cured in a fairly similar way. Bacon is smoked, which gives it a signature flavor that is deep but quite different from the Italian meat. One of the best ways to get around this issue is to boil or blanch bacon in water for several minutes. This can be fairly brief, so that the bacon is not cooked, but the smoke flavor is drawn out of it into the water. The bacon can then be used as a substitute for pancetta in any dish, and should be treated in whatever way is called for by the recipe.
A kosher substitute for pancetta is a bit more difficult to find, however, since pork meat of any kind does not work. Goose or duck may be the best option, since it can have fat content fairly close to both American and Italian bacon, which is important to most dishes that include them. Cured duck or goose may be available in some areas, and can provide a great deal of similar flavor. There are also types of cured beef, such as pastrami, that can work quite well as a substitute for pancetta when pork is being avoided.