Simple Currency Converter is a very handy currency converter. It sits either on your Dock or the Menubar and it can provide quick conversions between lots of currencies.
When you click on the application's icon, a new window pops up. This window displays a few currencies by default, but you can add new ones if you want. I don't think there is a limit of currencies that can be displayed. Each currency has a big circular flag next to it, which should let you recognize them more easily. Under each currency listed, you will find a text field. Here, you can type in the amount of that currency that you want to calculate. When you do that, you will see that all the other currencies show a different value. For example, by typing in "1.00" US Dollar, you can see for how many Euros, Argentine Pesos, British Pounds, etc can a Dollar be bought. Simple Currency Converter seems to be using Google's currency converter. I found that it is accurate enough, but there are always variations. For a more accurate conversion, you might want to check your banks listings. However, for quick conversions, this app works like a charm. I would love to have hotkey support, though.