With the cooperation of Sarofax and iPhone app developer Ash Mishra, I want to offer you TreeBook for free on iTunes. The app provides you with a pocket version of my tree leaf key at forestry.about.com.
Sarofax's TreeBook is leaf based and developed for beginning students unfamiliar with tree identification. It is designed as a beginning student guide using those leaved trees most commonly seen in North American forests. The app provides a system similar to the one on this forestry site for identifying the 100 most common trees with images, descriptions and links to more online information.
TreeBook supports a tree leaf key and provides a botanical glossary of the most common tree identifiers. TreeBook is especially helpful for identifying the most common Eastern U.S. hardwood trees.