I feed most of them back to the chickens, but have also used some in the garden. Last year I read in Mother Earth News that a mixture of coffee grounds and crushed eggshells would protect cabbage plants from cabbage worms if you mulched with it. So I dutifully did that, then started free-ranging my hens. You would have thought that I mulched the cabbage with chicken candy. First they ate the eggshells, then they scratched around all the coffee grounds, then they ate the cabbages. But at least I didn't have any cabbage worms!
I have heard that an old-timey fertilizer was to put crushed eggshells in a bucket filled with water and use that to water houseplants. Haven't tried that one yet though.