Well-capped beer stored below about 70 (50-60 is probably more ideal) will keep for long periods--a year easily. Colder temps retard the aging process.
Many beers will not taste very good after a year, of course. As a general rule, hoppy styles and lower-gravity styles taste better fairly fresh, while higher-gravity beers benefit from aging, perhaps a year or more in the case of barley wines, imperial stouts, strong belgian abbey-style ales, etc.
I find that most "normal" gravity brews are at their best after about 6-8 weeks in the bottle at cellar temps.