Mostly utilitarian stuff I've designed myself. (I'm probably in the minority of 3D printer owners in that regard.)

Just some examples of things I've printed:

• Custom mounts for hanging various things on the wall like lighting, a Raspberry-Pi-based NAS, and Nintendo Switch accessories to save floor/desk space.
• Custom shelf brackets for specialized cases (extending a not-level windowsill into a wider shelf for instance).
• My washing machine broke while it was full of water, so I partially disassembled the washer and made a drill adapter to run the pump off of a cordless drill before replacing the motor. Probably took 10 minutes to design and 25 minutes to print.
• A Raspberry Pi case that mounts under a particular desk without having to drill any holes into the desk.
• A decorative toilet paper holder.
• Wall spacers for mounting a pegboard. (Was quicker to find those on Thingiverse and print them than to dash to Lowes.)
• A complete set of all colors of lantern rings from the Green Lantern franchise. (A counterexample to the "utilitarian" theme.)
• A board game based on dominoes.
• A few gifts. A yarn bowl and sewing buttons. I also "wrapped" a small package once by designing and printing a custom box in which to present it. Seemed easier and more impressive than wrapping paper and Scotch tape.

Of the above, the pegboard spacers, lantern rings, yarn bowl, and some of the Nintendo Switch mounting items were not designed by me. The Raspberry Pi desk mount case, toilet paper holder, and gift wrap were adapted from or used elements from things I found online for free.

If you want to see the kind of things that people make with 3D printers, thingiverse.com is a great place to get a feel for that. All the models on there are free to download and print. Yeggi.com is an aggregator for multiple sites full of models both gratis and for pay. (Yeggi's homepage tends to have uncensored NSFW models, just FYI.)