Common Trees of Pennsylvania is produced by The Pennsylvania Department of Conservation and Natural Resources. You can use the links here to learn 57 of the 108 tree species found in PA.
Fantastic resource for learning more about your favorite trees is found in the Silvics Manual published by the United States Department of Agriculture, Forest Service, Agriculture Handbook #654.
The Anatomy of Trees has nicely labeled line drawings that show the leaf parts, types, and positions that are used as an aide in identifying trees.
A great way to identify the leaves of trees is to use the Key to Leaves of Virginia Trees. Answer a few simple questions by looking at your leaf and you'll be able to identify what kind of tree produced it. Just click on "I've got my leaf, let's get started!" and follow the directions. Once you get to the end of the key you will see a few pictures and find out more about your leaf and its tree.
The Arbor Day Foundation Tree Guide is known across the world for educating people about trees.
After you have learned to identify trees by their leaves, stop on over to the Arbor Day Foundation Web site and check out Carly's Kids Corner. Anyone can play their fun Leaf Miner game where you have to identify a falling leaf before it is eaten by caterpillars. It's an educational and fun way to learn about trees in your neighborhood!
An arborist's blog tells about people and trees. Some good up-to-date information on what's happenin' in the tree world.
American Forests is an organization of people devoted to the re-planting of our forests. Their Global ReLeaf efforts have already planted 20 million trees. Donations go to a worthy cause! You gotta read their 10 Reasons to Plant Trees — number one on the list is "saving time and energy"!
Trees are good! Now here's an informative site sponsored by the International Society of Arboriculture. If you need to cut down some trees or have other reasons to seek an arborist, this is the place you'll want to search. A couple of online brochures that everyone with trees should read include Proper Mulching Techniques, Why Topping Hurts Trees, and Benefits of Trees.