The last use may seem a little odd, but I swear, it's practical. Have you ever watered your pots, but been annoyed when the stream of water digs into the soil and exposes the roots of the plant? Use empty eggshells on the top to bear the brunt of the water and let it harmlessly disperse into the dirt.
The most common use for eggshells is to crush them up into dust or small pieces and use them to fertilize the soil. As they decompose, they will release calcium carbonate into the soil, which most plants need. The are especially useful for tomato plants.
If you crack your eggshells right, you can create nice little mini-pots to start seedlings. Just add some dirt and a seed to each eggshell.
Your new best friend in the garden.