Besides the main feed, there are a few supplements commonly fed to chicks, pullets, and chickens. Some are supplements that will improve their health and egg quality, and some are just yummy treats that they will gobble down.
Cracked corn. Chickens love corn, and cracked corn is a great way to lure them into the coop at night, or wherever you may want them to go. It also gives them some warm grain to digest on a cold winter night; the traditional story is that feeding chickens cracked corn in the winter evenings will keep them warmer overnight. It definitely gives them something to do as they love to scratch the ground for it.
Oyster shell. This is usually offered "free choice," meaning some is always available to the hens. The hens will eat it if they need what it provides, which is extra calcium, and if they don't, they will leave it alone. If your layer pellets contain calcium, you may not need this. But if your eggshells are soft, it's the first thing to try.
Crushed eggshells. Save their eggshells, dry them and crush them, and feed them back to the birds free choice for extra calcium. Some say this will make your chickens eat their own eggs.
Other food scraps. As mentioned above, chickens make great composting machines. You can save all vegetables except raw potato, fruits except citrus, and even meats for them. They will eat chicken, so that's your call. In nature they are insect- and even rodent-eating omnivores. Avoid feeding them beans, onions, and garlic.
Flax seeds. Some farmers like to supplement the commercial feed with flax seed for extra omega-3. You can do this, but be sure to feed whole flax because ground flax goes rancid quickly.
Grit. Coarse sand or dirt, called grit, is needed by chickens to use in their gizzard to grind any food outside their commercial rations. Just make sure the hens have access to small stones and sand and they'll be fine. You can also purchase grit at the feed store.
Cabbage heads. A popular winter trick is to hang a head of cabbage by a string from the rafters in the coop. The birds have a blast (as much as chickens can) pecking at the cabbage as it bobs around the coop. Fun times!
Other treat foods. Chickens especially like other dark, leafy green vegetables besides cabbage. Fruit is also a preferred treat food, especially berries (but not citrus, they don't eat it). They love garden compost like carrot thinnings, holey kale leaves, rotten squash, overgrown and seedy zucchini, and weeds (of course not sprayed with pesticides). Avoid mushrooms or other fungi, yew, and raw potatoes.