There are some instances where you don't need a passport, but it's usually between friendly countries that share borders for land entry. For instance, from what I've read Americans don't need a passport to travel over the border into Canada, but they do need other valid forms of ID, and a passport is easier, unless you have an Enhanced Drivers License from an issuing state (not all states have EDLs yet). Also in Europe if you're a citizen of a European Union country (EU National) you don't have to present a passport when going from one EU country to another - ie, A Finnish resident can cross into Sweden without showing a passport, but would if they went to Romania, as Romania is not a EU member. And it would still be a good idea to have it even in fellow EU countries in case of problems.