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 Dashboard, or by double-clicking on it from /Library/Widgets/.
Some users are reporting that other conversions are not working either. I did not have that problem, but if you do you can try deleting the ~/Library/Preferences/com.apple.Calculator.plist file (where ~ means your Home folder).
If you need to do currency conversions but don't want to use Dashboard, you can use Google instead; it will accept plain language conversions such as "20 euro to us dollars" and will offer other currency conversions as well.