The Economist magazine publishes the Big Mac index which "seeks to make exchange-rate theory more digestible. It is arguably the world's most accurate financial indicator to be based on a fast-food item."
According to Economist.com:
Burgernomics is based on the theory of purchasing-power parity, the notion that a dollar should buy the same amount in all countries. Thus in the long run, the exchange rate between two countries should move towards the rate that equalises the prices of an identical basket of goods and services in each country. Our "basket" is a McDonald's Big Mac, which is produced in about 120 countries. The Big Mac PPP is the exchange rate that would mean hamburgers cost the same in America as abroad. Comparing actual exchange rates with PPPs indicates whether a currency is under- or overvalued.
According to the magazine's January 22, 2009 print edition:
The dollar’s recent revival has made fewer currencies look dear against the Big Mac index, our lighthearted guide to exchange rates. The index is based on the idea of purchasing-power parity, which says currencies should trade at the rate that makes the price of goods the same in each country. So if the price of a Big Mac translated into dollars is above $3.54, its cost in America, the currency is dear; if it is below that benchmark, it is cheap. There are three noteworthy shifts since the summer. The yen, which had looked very cheap, is now close to fair value. So is the pound, which had looked dear the last time we compared burger prices in July. The euro is still overvalued on the burger gauge, but far less so than last summer.