What kind of tree is that? Pull off a leaf and snap a photo of it against a white background. Leafsnap will scan the outline and then return suggested species identifications, along with hi-res photos of leaves, bark, and blossoms to assist you in making a positive ID.
WIRED iPhone version offers games that help you learn to recognize common leaves.
TIRED Database currently contains only trees native to the northeastern US. Narrowing down the results can be tough.