It's not easy to give you a list, as this subject has a lot of sides to it. Many countries, such as the UK, allow multiple citizenship and make no distinction between one country and another. Others, for example Spain, have a special dual citizenship arrangement with certain other countries - in the case of Spain, the treaty is with several Latin American countries.

If you want to apply for US citizenship, you can see two lists of countries - one is of countries that would allow you to keep your citizenship after becoming a US national, and one that wouldn't. But this doesn't mean that the first list simply 'allows dual nationality' because in many cases it would depend on which other country you want to join.

There are some countries, like Germany, that don't usually allow dual citizenship except in specific cases, but these countries are in the minority. However, as the rules are so complex the only way to be sure is to ask directly or go to the embassy website of both your original country and the one you want to apply to.