Many wildflowers and grasses that grow in Minnesota are common in other parts of North America, too. Sites with photos and/or information that may be helpful are:
Flora of North America at eFloras.org: no pictures, but packed with detailed descriptive information on many plants, if you can interpret the botanical terms
Dictionary of Common Names of Great Plains Plants: What one site calls Field Sagewort, another calls Tall Wormwood. This site can help clear the confusion on common names if you have a plant's scientific name.
Plants for a Future: information on edible plants and medicinal uses, among other things.
Gardening with Natives
Recommended reading: Bringing Nature Home by Douglas Tallamy, on planting natives to sustain wildlife in your own yard.