Aeration stone. I had 4 kegs, and bought one for each keg to help carb my beer faster. I'd put hose on the air dip tube, and the stone on the end. And as long as the stone isn't clogged it probably does actually speed up carbing the keg. And the fun is also if the keg is under any pressure and you hook the air side on first pssssstttt, beer into your stone and up your air line, fun. Sometimes the hose would pop off and it's just floating in your keg now, oh the joys. And to clean the stone you can't touch it or the oil on your hands will clog the pores of the stone, but you somehow have to boil the stone for 20 minutes, in theory, they still ended up clogging on me. Just a massive pain for such little gain.
I've wondered if those bottle/carboy jet cleaners that you can attach to your sink are just junk. I can't ever see using something like that. If my bottle was that dirty I'd toss it. If my carboy is that dirty it needs to soak in PBW.