Let me know if any of these links are dead or if you find other good links that need to be on this page!
Colorado State University - Guide to Poisonous Plants
USDA - Agricultural Research Service - Poisonous Plants by Toxic Symdrome
International Veterinary Information ServiceYou need to register with the site but it's free and worthy of your free membership.
University of Illinois at Urbana-Champaign Veterinary Medicine Library Plants Toxic to Animals
Mount Lehman Llamas Poison Plants page - Good page of plants - Scroll down on the page
Cornell University Poisonous Plants Informational Database
Alberta Agricuture, Food and Rural Development
ASPCA: Animal Poison Control Center: Toxic Plants
Canadian Poisonous Plants Information System
Purdue University-Cooperative Extension Service
Poisonous Plants of North Carolina
The Prevention of Plant Poisonings in Livestock & Pets Program - Virginia-Maryland Regional College of Veterinary Medicine (VMRCVM) at Virginia Tech