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.
Usually I let them dry, then I crumble them up and feed them back to the chickens.
This year, I dried about 2 dozen shells and ground them in the blender. When I dug a hole for my tomato plants, I added about a tablespoon of powdered eggshell to the bottom of the hole. I heard the calcium is supposed to prevent blossom end rot. I'll let you know if it works.
I put ours in the composter or in the worm bins. We don't have any of our own shells, but we go through about 8-10 eggs/day in this house. Once the chix get to the laying stage, I'll probably dry, crush, feed. I'm not sure I like the idea of oyster shell...too much mercury.