For 2009 the Mets are starting what they're calling "Club Mets." No it has nothing to do with vacations in the Carribean, but it doesn offer Mets fans access to some deals and benefits.
It's $19.95 for the year and gets you access to MLB.com's Gameday Audio as well as some additional discounts and promotions. The MLB Gameday Audio alone is worth the price and if you take advantage of one of the other deals they offer it might be worth it.
For more info, visit Mets.com.