Great question! To my knowledge, the best bet is Brides Against Breast Cancer. The Making Memories Foundation sponsors Brides Against Breast Cancer events. Basically, you donate your wedding dress and it is then sold to another bride-to-be. The profit is used to make wishes come true for women and men with breast cancer.
So, not only is the program good for humankind, it is also good for the environment.
Here's what their website says:
This unique opportunity allows brides-to-be a chance to find the gown of their dreams at a remarkable price, while helping to make wishes and memory-making events come true. Most gown prices range from $99 to $799*, including hundreds of beautiful new name brand and designer gowns valued up to $8,000. *Exclusive designer and couture gowns valued up to $20,000 are also available at an incredible 25% to 75% savings. We gladly accept Visa, MasterCard, Discover, American Express and debit cards.Specific directions for sending in your gown can be found on their website.