Here are some great links on poisonous plants and substances (and other hazards):

• Toxic and Non-Toxic Plants @ The Budgie and Parakeet Place
• Gillian's Help Desk - Toxicity of different plants/objects/ substances pet birds might eat or be fed.
• Toxic & Nontoxic Woods for Birds @ MD Vaden of Oregon
• Safe Plants for Birds @ Hot Spot For Birds
• Suggested Avian Landscape Plants @ Exotic Edibles
• Hazards FAQ @ UpAtSix