It's easier being prudent with your travel budget when you know how much those souvenirs really cost. This free converter lets you compare the value of currencies against each other. It is ideal for tracking international exchange rates, but loses something of its value from previous versions, when comparing among currencies was more intuitive. This newer version also adds banner advertising; thankfully unobtrusive.
You can still swap the master currency and compare to monies from other countries, but to do so you'll need to tap the currency name--U.S. dollars, for instance--and enter the amount you'd like to set as your denominator--$1, $10, $100, and so on. You'll also be able to use Currency as a calculator by entering a precise value to calculate. Press "Done" to convert the sum from the master currency into another. Currency will also calculate the "inverse" of a sum.