What foods should a cat not eat?

There are many human foods that are toxic to a cat:
Chocolate - Can affect the heart and nervous system causing excitement / nervousness / trembling, vomiting / diarrhoea, seizures, muscle spasms, coma and death.
Caffeine - Can affect the heart and nervous system and causes the same or similar symptoms as chocolate.
Alcohol - Giving a cat alcohol repeatedly (or a lot of) will cause liver damage, breathing difficulties, coma and even death.
Garlic (any form) - Can damage red blood cells and lead to anemia.
Onion (any form) - Can damage red blood cells and lead to anemia. Onions are more toxic than garlic.
Tomatoes (all aspects of the fruit and plant) - Can cause the digestive, nervous, and urinary systems, but mostly causes severe digestive upset.
Potatoes (Raw and plant) - Can cause the digestive, nervous, and urinary systems, but mostly causes severe digestive upset.
Mushrooms - Some species of mushroom are highly dangerous as the can cause shock and even death.
Grapes and Raisins - It is unknown why these are toxic, but even a tiny amount can severely damage the kidneys.
Macadamia Nuts - Can cause gastrointestinal upsets, lethargy, vomiting and muscle tremors or stiffness.

Foods that are potentially harmful:
Liver (in large amounts) - A small bit of raw liver on occasion is very beneficial to a cat.
COOKED bones and fish bones (including raw and cooked) - Cooked bones are very hard and brittle and will splinter which can cause obstruction or laceration in the digestive system. Raw bones are much safer as they are soft and do not splinter.
Dog Food - Dog food is vastly different to cat food so doesn't have the added Vitamins and Fatty Acids cats need, such as Taurine.
Foods high in Salt and Sugar - Too much of both can harm your cat's health (sugar can cause obesity and diabetes and salt will cause a chemical imbalance, as well as dehydration).
Milk - Cow's milk, despite popular belief, is not tolerated by many cats as many are lactose intolerant. An adult cat gets no nutrition from any sort of milk, but specially formulated cat or kitten milk can be used as an occasional treat.