The Big Mac Index from The Economist magazine. Be sure to read the Oct. 14, 2010 summary, which is key to understanding real exchange rates.
There is also a video clip explaining the basic concept.
Why Real Exchange Rates? by Luis A.V. Catao, Finance and Development,44:3, Sept. 2007, starts from the Big Mac Index to provide a nice brief explanation.
Burgernomics: A Big Mac Guide to Purchasing Power Parity
by Michael Pakko and Patricia Pollard, St.Louis Fed Quarterly Review , November 2003.
WARNING: Set PDF Advanced Printing options to PRINT AS IMAGE or your output will be unreadable.
Methodology for CGER Exchange Rate Assessments by Jonathan Ostry et al. at the International Monetary Fund. CGER is the IMF's "Consulative Group on Exchange Rate Issues" You should review the two most interesting approaches: Macroeconomic Balance, focusing on the stability of the current account (flow) and the External Sustainability Approach, focusing on the sustainability of the net foreign asset position.