Eggs. If we have eggs we are not sure when they were laid, we just toss them into the run and they mob around and gobble them up shells and all. They do not seem to care if it is cooked or uncooked. This first happened when we dropped an egg when collecting. They immediately swarmed it and gobbled it all down in a matter of seconds.
Salmon Skin off the barbeque. They love it.
We once accidentally left a trash bag outside, the chickens got into it and ate anything they could get down their gullet. Kitty litter, confetti, every bit of food. One tried to eat a piece of wire, but I pulled it back out. They are kind of like goats I think.
My grandfather used to put a kernel of corn on a string and let a chicken eat it. Then he would pull it back out and the chicken woudl eat it again, over and over. I guess that was Iowa entertainment for kids in the old days.