Exercise for overweight or obesity. It's in the form of a Cochrane Report. The Cochrane Collaboration is an international, independent, not-for-profit organisation of over 27,000 contributors from more than 100 countries, dedicated to making up-to-date, accurate information about the effects of health care readily available worldwide. It is a thorough review of the evidence in the area of obesity and exercise and its about as good a review as you will find anywhere. The report is long, detailed and extremely useful. I refer to it frequently and i'm sure i will do in the future too! The report can be found here.
Shaw K, Gennat H, O'Rourke P, & Del Mar C (2006). Exercise for overweight or obesity. Cochrane database of systematic reviews (Online) (4) PMID: 17054187