Leafsnap is a project I've been working on for about 2 years and we finally launched yesterday! It's a free app for the iPhone (with an iPad version coming next week and an Android version later in the summer) that contains very high-quality images and descriptions of the tree species of the Northeast US. It also has an automatic-recognition feature that uses computer vision to help users identify trees by taking a photo of a leaf.
This project is a joint collaboration between the computer science departments at Columbia University and the University of Maryland, and botanists at the Smithsonian Institution.
Please let me know what you think! Also, I'd be happy to answer any questions about the app or the recognition technology behind it.
Website: http://leafsnap.com/
iPhone app: http://itunes.apple.com/us/app/leafsnap/id430649829?mt=8