The stock Calculator in OS X has a ‘hidden’ unit converter feature. You might be delighted to discover that the app can perform unit conversion for currency, ounces to grams, kilometres to miles, and many more.
To convert let say Euro €50 to US$, enter ’50’ into the calculator. Select from menu Convert > Currency… Click on the “Update” button to refresh the exchange rate. Select “Euro” for From field and “US Dollar” for To field. Click on the “Convert” button to get the US$ value of 50 Euro.