The ASPCA Animal Poison Control Center lists the following foods as toxic to cats.
Onion and Garlic
These vegetables have a compound which destroy the red blood cells in cats causing a specific type of anemia called “Heinz body anemia.” While onions seem to be more toxic than garlic, both should be avoided.
Foods and drinks such as chocolates, cocoa, cola, and black teas contain a chemical called Methylxanine. When ingested, Methylxanine can cause vomiting, diarrhea, excessive thirst and urination, abnormal heart rhythm, tremors, seizures and death.
This fruit contains a bitter, poisonous alkaloid called Glycoalkaloid Solanine which can cause violent lower gastrointestinal problems. The leaves and stems of the plant is especially toxic.
While not toxic, some cats are lactose intolerant causing stomach upset, cramps, and gassiness
Grapes and Raisins
While this has been found mainly to be toxic to dogs, the ASPCA advises not giving these to any pet in any amount. There are many unknowns with the toxic potential but these fruits have been known to cause kidney failure and death.