The Arbor Day Foundation has a terrific website that can help you identify trees out in the field.
You can either buy their guide What Tree is That? or you can use their online tree identification site.
Their site has tons of other tree-related information. Go check them out!
You can also identify scores of trees by their common name at the Ohio Public Library Information Network.
Indiana Woodlands Steward has a nice site, and the Nature Conservancy has a great page on identification in Winter.