Toxic substances
A cats liver is not as efficient as a humans, consequently many familiar plants and substances are toxic for cats. The most important ones to know about are; paracetamol (Panadol, Tylenol), aspirin, chocolate, lilies, and tea-tree oil. There are many others, check out the links below.
http://cats.about.com/cs/catfood/a/humanfood.htm
http://www.messybeast.com/teatree.htm
http://www.cat-world.com.au/cat-worldtoxic.htm
http://cat-care.suite101.com/article.cfm/aspirin_and_tylenol_are_toxic_to_cats
http://www.moggies.co.uk/plants.html