Northern & Central Midwest
The Ladybird Johnson Wildflower Center
The Web site of the Ladybird Johnson Wildflower Center at the University of Texas has a native plant database, image gallery, and how-to articles on using wildflowers and other native plants. Best of all, even though it's based in Texas, you can select your region and call up plants that are strictly Midwestern. This is a truly classy, well-presented site.
What is Invasive and What to Substitute
With the great debate these days over what is native, what is not, and what is invasive, Native Alternatives to Invasive Plants, by C. Colston Burrell (Brooklyn Botanic Garden, 2006; $9.95) is a good handbook to help you decide for yourself what you should be using in your garden. It may be hard for some of us to give up our burning bushes, which are now listed as invasive, but this handy book will help us find a good substitute. The book also includes federal lists of invasives by state.