Showbizguru is right. Most beers should be drunk fresh; some, however, do develop more complexity with aging. These are big, intense beers such as Belgian strong ales or English barleywines (or beers made to those styles in other countries, of course).

Cool and dark are two key things to consider when looking for a place to age a bottle of beer. There's a good how-to guide to cellaring beer on the Beer Advocate website.

How long a beer will keep depends on the beer as well as the cellaring. I've had a twenty year old Thomas Hardy Ale that was magnificent, and recently shared a ten year old bottle of Belgian Oude Geuze which was superb. But cellaring is a bad idea for beers like pale ales or IPAs because one of the things that happens as a beer ages is that the flavor of the hops mellow and become less prominent. For example, after a couple of years, no matter how carefully stored, Stone's Arrogant B... Ale will be a pretty mellow dude.