AGRICOLA - database of literature from the National Agricultural Library.
BONAP Checklist - a synonymized checklist of the vascular flora of the U.S., Puerto Rico, and the Virgin Islands.
Flora of North America - names, characteristics, species for over 21,000 plants.
Index Nominum Genericorum (ING) - "a compilation of generic names published for all organisms covered by the International Code of Botanical Nomenclature."
Noble Foundation Plant Image Gallery - images of more than 600 species to assist in identification.
Vascular Plant Image Gallery - from Texas A&M.
WWW Virtual Library of Botany - WWW sites.