I have several that I keep the special Easter eggs in. If I had children I would have made a project of decorating the boxes in an Easter theme. I only need a few for this however. I've also read of firestarters - fill the compartments with drier lint, and pour in melted wax/candle ends. Haven't done it though.
They are also good for people who raise feeder crickets, roaches, etc. (The insects like to hide in them so they are popular for use in various habitats). You might try pet stores or place an ad on Craigslist for people who keep these types of insects?
I gave a bunch to a young man who works at a golf course. He goes into the water hazards and retrieves lost balls, also in the rough. He is allowed to sell the balls he finds so he cleans them and packages in egg cartons. Also our local weekly farm auction will sell stacks on commission. You don't make alot of money but it helps pay gas to get to the sale!! When I was teaching I got alot of cartons donated to me but now that I've retired the flow has dried up so now we get only a few from friends that we use for our own eggs.