Cornell University
Cornell University: Plants Poisonous to Livestock
A site dedicated to poisonous plants. Supplies names and information on each plant as well as frequently asked questions about poisonous plants.
Colorado State University
Colorado State University: Guide to Poisonous Plants
A database that is very helpful when searching for info on different reactions to the poisons in the plants - mainly for animals though.
Other
Botanical.com: Index of Poisionous Plants
This site from Botanical.com provides an index of poisonous plants with in-depth descriptions and symptoms.
Texas A&M University
Texas A&m Agrilife Extension: Common Poisonous Plants
An article on common poisonous plants and plant parts.
Other
Poison and Drug Info. Center: Poisonous Plants
This illustrated catalogue of poisonous plants includes a brief description of each, lists poisonous parts and the symptoms of human contact.
Other
Florida Gardener: Poisonous Plants of Florida
A list of poisonous plants in Florida and how to deal with them and their effects.
Science4Fun
Science4 Fun: Poisonous Plants
Learn interesting facts about poisonous plants including why they carry poison and how they are used. Also describes some common poisonous plants.