Botany Encyclopedia of Plants.
Provides general description of plants, various methods of cultivation, soil and temperature requirements, pruning.

Virtual Palm Encyclopedia
History, horticulture, pests and diseases, conservation, photographs, and references.

Metropolitan Plant Encyclopedia
Consists of a series of comprehensive pages on the plants of the New York metropolitan region.

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.

Lapshin's Succulent Encyclopedia
Photos of succulents of numerous families.