The built in "Stocks" Application on iPhone can be configured to show currency rates in addition to stock rates.
To get the desired currency rate to the stock applications

1. Run Stocks App and tap on the "i" icon on lower right corner.
2. Then tap on "+" button which is supposed to add stocks
3. In the window type currency equation in format AAABBB=X (AAA is a currency symbol and BBB is one)
4. Eg: USDINR=X For Currency rate of US Dollar to Indian Rupee.
5. When you refresh you will have the desired currency rates also displayed