USDA PLANTS Database
The PLANTS Database provides standardized information about the vascular plants, mosses, liverworts, hornworts, and lichens of the U.S. and its territories. Search over 40,000 plant images of US plants.
Botany.com offers an encyclopedia of flowers and plants and resources to help people learn how to identify any different kinds of plants.
This is a good reference for looking at the plant kingdom.
Other Botany Reference Links:
Virtual Library of Botany
Look at various botanical resources coving everything from Agriculture to Biophysics, sponsored by the University of Oklahoma.