Arvind writes, "I have recently acquired US citizenship but my Indian passport has a visa valid for Argentina for another several years. Can I use it to travel to Argentina or do I need to pay again for a new visa?"
First of all, when you acquired US citizenship, unless a country has dual citizenship (India does not), you no longer have a valid Indian passport. If you read the fine print, all visas must be presented with a valid passport for the person to whom it was issued. In any case, visa requirements vary by nationality. For example, in the past you would have needed a visa to visit France, but not any more. Accordingly, you will need to pay again at the Buenos Aires airport to get a visa-on-arrival.