Leftover oatmeal from kids' breakfast is my favorite way to sneak shells into their food!
I toss all the eggshells in a big yogurt or cottage cheese tub I keep on my counter, and when it's full I toss them into the toaster oven (not in the tub - on the oven's cooking sheet). I try to use the leftover heat after making a mini-pizza or something like that. Heating the shells dries them out and makes them easier to crumble.
My current way to crush the shells: I have a plastic peanut butter jar with a rock in it. I shove them into the jar and shake the rock. Not as fine-grained of a crush as some of the other things I've tried, but it's cheap, and the chickens haven't complained yet. It gives me something to do when I wait for the microwave. And it makes my wife laugh.