The National Garden Association hosts an easily referenced weed library at Garden.org. Each entry includes pictures for plant identification, information about weed control and management, and warnings about plants that are potentially toxic to humans.
Some regions have weeds that are not common nationwide, but which present more localized problems. The U.S. National Arboretum website offers links to state-by-state information about weed control. There are also some weeds whose control is mandated or impacted by local, state or federal legislation. Invasive.org and the Invasive Plant Atlas both offer a list of some of the most common invasive species, including clear pictures to assist in correctly identifying the weed plant, as well as information about weeds that are declared noxious by state law.