See also sources of plants and seeds which includes plant finding databases, and useful plant profiles, which includes numerous books and article collections.
AgroforesTree Database
Species info and selector from World Agroforestry Center, great fodder, nitrogen fixing, and erosion control species.
Apios Institute for Regenerative Perennial Agriculture
User-generated database of useful perennials, polycultures, and food forests for cold climates.
Bosque Comestible Project
Spanish-language user-generated database of useful perennials, polycultures, and food forests for Mesoamerica and Caribbean.
Cornell University Poisonous Plants Informational Database
Searchable database of poisonous plants.
Fire Effects Information System
Incredibly detailed information on response of wild plants to fire, with more great ecological information. Mostly western US.
Food Plants International
Incredible global database of 18,000 edible plants.
Native Plant Database
Search for native plants of US and Canada.
The Plant List
Correct scientific names and synonyms for almost world 300,000 species.
Plant Resources of Tropical Africa
Awesome database of useful plants of sub-Saharan Africa.
Plants for a Future
I use this all the time. Search over 7,000 useful plants for temperate climates.
Survey of Economic Plants for Arid and Semi-Arid Lands (SEPASAL)
Searchable database of useful plants for tropical and subtropical drylands.
Tropical Forages
Excellent selector for a range of climate, soils, and purposes.
USDA Plants
I use this every day. Great info on native and naturalized species, including range maps of US and Canada. Great pdf profiles for many species as well.