I was delightfully-surprised to find how easy it is to use cracked egg shells to start seeds. My biggest challenge was to remember to save them instead of adding them to the compost. After marking empty egg containers on the outside, I started to save broken shells inside egg containers and storing them on one of my garage shelves.
I wasn't sure how well they would work with delicate seedlings so I started with tomatoes. After watering and keeping them warm, tomato plants sprouted easily, including growing through the shells when tomato plants became too big for the shell. To plant, I dug a small hole and put plant and shell in, then covered.