The two DS carts shown below are "Download Station" carts. When you go to a videogame store and download demos onto your DS, it's because they have a DS in the back with one of these carts in it, "serving" the demos wirelessly. Nintendo makes the stores return these when they're done and they aren't supposed to fall into public hands, but like anything else, a few slip through the cracks.
I'll feature some other dev/proto/demo stuff as time goes on. I've got loads of it.
Anyone potentially interested in this stuff should read THIS.