There is a great app for smart phones developed by Columbia U, U of Maryland, and Smithsonian Institute called Leafsnap. http://leafsnap.com
Basically, if you have allergies and are wondering if you are allergic to the tree just outside your window or anywhere you spend time, you can take a photo and LeafSnap will give you name of the tree. Although initially geared to East Coast, site now growing to include West too.
The site is useful for user (allergy sufferers) and but is actually also a research tool – photo you take is downloaded to scientists and helps researchers know what plants are growing in different regions of the United States.