Don:
Thomas Hardy's Ale by the fine folks at O'Hanlon's Brewing Company in Whimple, Devon, UK. It's an 11.5% ABV unpasteurized barleywine, aged 25 years. It will continue to mature if you can keep from drinking it. It has a very strong mead/molasses flavour to it. Simply wonderful. If you want more recommendations, I have a
bunch.
crosscountry: You're only partially correct. Most mass-market and pasteurized beers will go bad in 3-6 months. Other beers (unpasteurized barleywines, vintage ales, etc.) will benefit from further bottle-conditioning. They should be kept in a cool, dark area for a period depending on the specific beer. Unpasteurized beers will change the most profoundly, since the yeast is still active. More info
here.