FXRates

FXRates is clean and simple to use and there are no background services that eat up your memory and CPU. It doesn't force you to set a single base currency for your conversions or rates but lets you monitor any number of currency pairs right from the main screen. Simply click on any item to open a converter and click on charts to see detailed historical data about any of those currencies and swipe down to update. You can easily add remove pairs from the menu or from the add button.