I've seen releases of doves and butterflies at a few outdoor weddings. They're beautiful, but I think this new tradition from the Pacific Northwest beats them both for symbolism.
The wedding takes place in a quiet cove near a pond, lake or river. Once the happy bride and groom are reunited they release a bevy of frogs (native species only, please) toward the water.
Finding her prince
The frogs hopping toward home symbolize the bride and groom leaping into their new lives together... Not to mention the old "non-princes" and "non-princesses" from their past that the bride and groom are happily letting go of.
It's a burst of fun for family weddings, and of course it's popular among environmentalists - it's definitely another way to have a green wedding!