Here it is - the Simple Currency - converter with really easy-to-use interface and which does it's task perfectly!
You are able to choose from 83 world currencies. By default the selected ones are EUR, USD and GBP.
If you often convert from one exact currency to another, you can easily select Default Currency from the settings.
You can also configure automatic (while typing) or manual (clicking a button) conversion.
It has an offline mode, but before using this, you need at least one refresh for a particular currency pair (from-to).