Northern & Central Midwest
Illinois' Best Plants
This Illinois' Best Plants is a wonderful site for searching for plants. You can enter the criteria you want, such as color, bloom time, and height, and it will give you a list of choices. Each choice has a full-blown fact sheet with photos. Even though the database is centered in Illinois, you can enter a hardiness zone and find plants that will work anywhere in the Midwest.
The American Horticultural Society Flower Finder by Jacqueline Heriteau and Andre Viette (Simon and Schuster, 1992; used copies available) is a great place to start with perennial garden design. It is a comprehensive, easy-to-use guide to selecting perennial flowers. There are over 400 photos, and lists for every possible combination. It also covers annuals, bulbs, and herbs, and many of the lists are set by site selection. This is a great tool for making good choices of plants adaptable to hot and dry situations, shady gardens, wet gardens, etc.