Currency is a great free app for Android created by Jeff Grossman. It is incredibly useful when you are travelling and still very handy to have even when your not.
When you start the app you see a few countries currencies and its very easy to add more. The rates update as soon as you launch the app, ensuring the latest exchange rate. 
To add more currencies, press the menu key, then Add Currency. The country is added alphabetically to your list.
You can touch a currency and enter a number, it will then convert across all te other currencies listed. Very handy when you are traveling and need to convert foreign currencies to your home currency.
At the top of the screen it will say when the last update was, you can start another by touching menu, Refresh Rates. At the bottom you will see a small ad, which is actually a good thing. This is keeping the app free and allows you to add as many currencies as you like for free.