Leafsnap is the nature companion of the future. Take a picture of any leaf and the app uses face-recognition-like software to identify the tree it came from. It also networks the GPS coordinates, giving arborists a clearer picture of what's growing where. The inventory is currently limited to trees in the northeastern United States, but a national database is in the works. Free; iPhone only.
BEST ANDROID ALTERNATIVE: Audubon Trees; $10.