Household plants, especially Philodendron. Many garden plants are poisonous, especially Rue, Privet and Laurel berries, Yew berries, and Laburnum seeds. Paint thinner and paint remover, petrol, paraffin, and metal polishes. Keep them locked out of your child's way in the garage, and don't let him into the workshop unsupervised. Lock them away.
Tobacco can be lethal to a child.
One cigarette could kill a one-year old, if swallowed, and a large swig of neat spirits could kill a toddler. Read the labels on all household products before buying them, and try to use the least-toxic ones. Lock up all medicines and harmful substances. Secure all cupboards that contain poisons, even those you might assume to be out of range. Poison experts have seen plenty of toddlers who dragged a chair over to a kitchen counter, climbed onto the counter or even the refrigerator, and opened an eye-level cupboard.