LeafSnap is a new iPhone app that uses facial recognition techniques to identify trees based on photos of their leaves.
Leafsnap contains beautiful high-resolution images of leaves, flowers, fruit, petiole, seeds, and bark. Leafsnap currently includes the trees of New York City and Washington, D.C., and will soon grow to include the trees of the entire continental United States.
Wow. Garden Design has more info. (thx, claire)