Botany Encyclopedia of Plants.  
   Provides general description of plants, various methods of cultivation, soil and temperature requirements, pruning.
   http://www.botany.com/

Virtual Palm Encyclopedia  
   History, horticulture, pests and diseases, conservation, photographs, and references.
   http://www.plantapalm.com/vpe/vpe_index.htm

Metropolitan Plant Encyclopedia  
   Consists of a series of comprehensive pages on the plants of the New York metropolitan region.
   http://www.bbg.org/sci/nymf/encyclopedia/

A Modern Herbal  
   Index of more than 800 varieties of herbs and plants, word search of the text, recipes, and an index of poisonous plants.
   http://www.botanical.com/botanical/mgmh/mgmh.html

Lapshin's Succulent Encyclopedia  
   Photos of succulents of numerous families.
   http://www.lapshin.org/succulent/