A cat that's both curious and hungry will annoy you to death if you are sitting on the couch with a bowl of soup, or any other meal. It's tempting to share just a bite with your beloved feline, but stop and think before you do. Cats have very particular metabolic systems, and some foods we eat are downright dangerous to them. Even sushi. Sushi? Well, sashimi to be exact. Raw fish can be harmful to your cat. But what about feral cats, who eat fish all the time? You have to remember that feral cats have short lives, and often no choice as to what they eat. In the wild, it's a roll of the dice, and to an animal, eating is always better than not eating. You can do better by your cat.
Lifehacker has a list of foods that are downright dangerous for cats. It's accompanied by a list of foods that are okay for cats, but keep in mind that too much of the same food will fill your cat up, but may deprive him of complete nutrition. They best rule of thumb is to stick with cat food, which in most cases is formulated to be be nutritionally complete.