BONAP North American Vascular Flora: http://www.bonap.org/
A herculean effort by the Biota of North America program to catalog the entire vascular flora of North America. The Taxonomic Data Center allows species searches by non-dichotomous keys of the most accessible characteristics.
Name that Plant: http://www.namethatplant.net/index.shtml
Information on native and naturalized plants of the Carolinas and Georgia.Includes links to a large collection of field guides and books on native plant gardening. Online key is rudimentary, but photography is excellent; so if youthink you know what your plant might be, this is a great place to check.
Michigan Flora: https://www.michiganflora.net/
Keys, maps, pictures about native plants and information about alien species.
USDA Plants Database: http://plants.usda.gov/adv_search.html
The advanced search function of this database allows searches for a number of characteristics at once and provides images and much other information on each species. It's also a great way to help writers think about what plants might be present wherever within the US they may wish to place their stories.
Tree Identification Videos with Don Leopold: https://www.youtube.com/playlist?list=PLBE1197A3397CAE00&feature=plcp