This is an identification program for wildflowers in the northeastern and north-central parts of United States and adjacent Canada (if you live on the West Coast, try Reny's Wildflowers). Fill out the form as best as you can and you'll find which wildflowers match. It's better to start with a few characteristics you are sure about, then add more as needed. Use the "Back" button to go back and add (or subtract) characteristics. If you want to see all the flowers in the database, simply hit "Identify" without checking any boxes.
Some books that can help include:
Wildflower Leaf Characteristics: see this image (from Wikipedia) for a useful description of the leaf terminology used below.