best free desktop gadget to get exchange rates
Calculator.app doesn't display the exchange rates, but it does allow you to convert between them.  You can do this by entering a number in the calculator, and then going to the Convert > Currency... menu and choosing the appropriate currencies.

If you're looking for a Dashboard widget to show exchange rates, I'd suggest this one:  http://www.apple.com/downloads/dashboard/calculate_convert/currencyconverter_palplesoftware.html