A 3d printing company that you can sell your stuff through. They will also do your fabbing for you if you don't have a 3d printer.
Link goes to marketplace because otherwise this very useful thing is hard to find.
They make custom case badges, labels, and inventory tags.
This company makes custom business cards out of metal. Even lockpick ones.
MPC is a company that prints custom decks of cards (54-234 cards per pack) in multiple sizes. You'll probably want to buy in bulk to get a decent price. People tend to use this service for their aftermarket Foo Against Humanity decks.
Silver Ring Splint manufactures custom hand and finger splints for Ehlers-Dahlos Syndrome and arthritis. Looking at them, you can't even tell they're not exquisitely designed custom jewelry.
A relatively simple yet useful online pulsar map generator. Displays the Pioneer and Voyager map by default but can generate custom maps with a bit of fiddling.
Send them a graphic of some kind and they'll do custom fabric of it for you. No minimum order, they even sell test swatches.
A company that sells keyboards, keycaps, keyswitches, cables, and all the other nifty-keen-like-wow stuff that people use to trick out their hardware.
A company that prints custom playing cards in small batches. Will even print single decks of cards at a good price. Download their template file prior to sending an order.