An item missing from the core set of Apple iPhone utilities is a Currency Converter app built-in. As you can see from this iPhone screen capture, using the Stocks app you can track daily currency fluctuations just like stocks:
1. Go to the stocks app
2. Add a new stock
3. Use the format Currency symbol 1 plus Currency symbol 2 (without space between) =X (example to convert US Dollar to Canadian Dollar would be USDCAD=X or for the Euro it would be USDEUR=X)
A nice thing about this approach to tracking currency information is daily rates will be listed with other stock market information and you will see the daily +/- change, a day/month/year history of the currency plotted on a graph just like other stocks in the Stocks app.
Add as many world currency entries as you want. You can get currency symbols here or do a search for world currency symbols in Yahoo or Google.