There are tons of eggshell uses out there.
Crushed eggshells can be sprinkled around your garden to keep certain pests (especially soft-bodied pests like slugs) out. Or, you can mix the crushed shells with your soil or compost and add nutrients back to the soil. The extra calcium in the soil can help prevent blossom rot in your tomatoes!
Do you keep chickens yourself? Crushing up the eggshells and feeding them back to the chickens helps keep their calcium levels up- which means they aren't depleting themselves making eggs day after day. You can also head off potential health hazards like soft shelled eggs that might get stuck inside the hens.
Some folks add crushed eggshells (not powdered) to their coffee before they brew it, to help remove bitterness.
My dogs love to eat eggshells and it gives them a calcium boost. You can also grind them fine with a mortar and pestle and put the powder over your pet's food, or do as I do- just give them hard boiled eggs, slightly cracked, with the shell still on!