Northern & Central Midwest
U.S. Army Center for Health Promotion and Preventive Medicine
This list is by no means exhaustive, but the U.S. Army Center for Health Promotion and Preventive Medicine's Guide to Poisonous and Toxic Plants is a good place to start. The key is being able to identify the plant in the first place, so you may need to go to a couple of different Web sites to find photos and more information. This site does tell you what the symptoms are as well as what parts of the plant are toxic. As an example, rhubarb stalks are not poisonous but rhubarb leaves are.
Favorite or New Plant
Lemon Queen Sunflower
Helianthus 'Lemon Queen' is a stately perennial sunflower reaching six to seven feet. The stalks are covered with lemony single daisies from July through frost. This plant takes virtually no care, will withstand drought, and is tolerant of just about any soil. It is perfect for the naturalistic garden, and combines beautifully with boltonia, tall asters and goldenrod. This is a butterfly draw, and if you want it to reveal itself at a lower height, simply prune back in June.