This app drew its exchange rates from the Yahoo Finance API, which has since been killed. I'm not aware of any other free API that can provide the necessary data, so this is effectively a dead project.
This app converts a price the user enters in a fiat currency (US Dollars, Euros, British Pounds, etc.) into the equivalent in sound money--gold, silver, and Bitcoin. It also displays a list of commonly available gold and silver coins that match, as closely as possible, the specified fiat price. For example, if the user enters a fiat price of 50 US Dollars, it will show:
3 one oz silver coins
1 silver dime
1 silver quarter
0.26 US Dollars remaining
Currently this app is Mac only. It was intended to be cross-platform (Windows and Linux as well), but a Xojo bug got in the way.