-www.makingmemories.org - in their "Brides Against Breast Cancer" campaign, MakingMemories.org hosts trunk shows all over the country featuring donated gowns, so brides-to-be who have been diagnosed with breast cancer can still afford the dress of their dreams at a fraction of the price.
-www.idofoundation.org - at the I Do Foundation's website, you can do more than donate your dress. For example, you and your fiancé can register with select retailers that will donate a percentage of the profits from gifts purchased by your guests. They even offer resources for planning a "green" wedding.
You could also sell your dress to a consignment store and donate the money to a charity. And while you're at it, ask your bridesmaids to donate their dresses too, to a charity like The Glass Slipper Project.