OS X's built-in Calculator.app has built-in support for currency conversions. To get accurate results, you have to update the currency exchange rates from inside the application.
This feature is currently broken for many users, as I discovered after it wouldn't work for me. Several threads on the Apple Discussion forums show this is a "known issue" with no known fix at this time. This seems to be happening to users of both Leopard and Snow Leopard.
In the meantime, you can use the "Unit Converter" widget which is still working and updates successfully. You can add the widget by clicking on the "+" in Da
