I remember reading that garlic, onion, and fish should not be given because it can cause off flavors in the eggs. Also they dont digest raw potatoes but cooked is fine. Dairy items should be given in small amounts, but i dont remember why. I am gonna have to get out my Storeys chicken book again.
From experience, i know they love any surplus or scraps from the garden. Sometimes when those tomato worms get in there and eat wholes through some tomatoes, somehow that is enough to gross me out as to where I pick the half eaten tomatoes and give them to my chickens. They dont mind that they are getting to eat after them worms, oh, and if I DO happen to find that tomato worm himself or any other pests from the garden, they are given as tasty treats to my chickens. Even slugs, they say yum.
And a favorite treat that is cost efficient is a handful of catfood to a few chickens. I believe I was informed of this through Storeys book again. As a bonus, the higher vitamin A in the catfood prevents blood spots in the eggs. It is true because I dont get eggs with blood spots.