In my Biology class back in high school, we were assigned a leaf identification project. We had to collect fifty different types of tree leaves and identify them.
I recall having a tough time figuring out what some of the mystery leaves were, myopically combing through vague leaf illustrations in my identification guide book and trying to find one that looked at least a little like the weird-ass mutant leaf I was holding.
Of course, this was back in the Stone Age before we had The InterWeb and all its fancy searchamadealies. If only I had access to something like "What Tree Is That?" via The National Arbor Day Foundation, which is available in both Eastern/Central US and Western US flavors. There's even a little flash animation tutorial to further help.
Speaking of trees, do you know what a dendrochronologist is? It's someone who studies tree rings. You know, the way you tell how old a tree is. Catch some dendro-citement at The ULTIMATE tree-ring pages! (gratuitous capitalization and punctuation left intact)