Northern & Central Midwest
The University of Illinois Urbana-Champaign Weed Identification Web site is one of the most helpful because it allows you to enter the characteristics of the weed and basically key out which weed you have. The photos are excellent, and include not only the leaf and flower, but also the weed as a juvenile so you can identify and eradicate it early in the lifecycle.
Safe Weed Control
The Gardener's Weed Book: Earth-Safe Controls, by Barbara Pleasant, is a great guide to using safe methods to control weeds. She describes the life cycles of many Midwestern weeds, and gives straightforward information on how to control the pests naturally. This is a great addition to the green bookshelf. The pictures are drawings, but use this in combination with some of the great Web sites and you have a complete tool.