Leafsnap for iPad

This free mobile app uses visual recognition software to help identify tree species from photographs of their leaves. Leafsnap contains beautiful high-resolution images of leaves, flowers, fruit, petiole, seeds, and bark.
Developed at Columbia U, U of Maryland, and Smithsonian. Note: recognition feature only available on iPad2. You can create your own gallery of leaves.
Here is the link via App Shopper.