Anything that custom-fits something. I do a bunch of functional prints, mostly things that are specific to something I own. I printed my wife a phone holder sized to fit her phone and case, I printed wall mounted holders for my remote and keyboard for my HTPC, and I recently upgraded to a larger printer that I used to create custom drawer organizers that fit exactly the things I need. I've also used it to print replacement parts for things, replacements for missing pieces, etc.