Vanderbilt University's tree ID site couldn't be any easier to use for trees common to TN and similar climates.
Check it out at
For Native & Naturalized Plants of the Carolinas and Georgia and similar climates, go to this link
at Name that Plant.
In or near Minnesota? Check out Key Plants appearing in the Field Guides to Native Plant Communities of Minnesota: Forests and Woodlands.
Northeastern Shrub and Short Tree Identification online book is at http://www.esf.edu/ivm/PDFs/ShrubID%20preview.pdf
Trees of Texas: Tree Identification 101 is at http://www.isatexas.com/images/pdf_files/TreeID/Key_to_Texas_Tree_Species_TFS.pdf
The Huachuca Audubon Society has a tree key for AZ and NM at
You get the idea - there is a tree ID website for each of us!