Some Belgian beers, notably Lambic, are brewed with hops that have been aged to drive off some of the more aggressive fresh hop aromas that are desirable in many other beer styles. The result is a hop which has a preservative effect on beer, but without any significant flavor contribution.
Making your own aged hops
Unfortunately, aged hops are rarely available to home brewers. If you do not have access to aged hops, you may want to age your own hops, or pre-treat them in a way that simulates this aging. This can be done by heating them in an oven over low heat (150 degrees) for an hour or longer; some sources suggest up to 12 hours. Be aware that this may also create a significant odor as the volatile oils and acids are driven off of the hops into your kitchen.