There's a lot of obscure/weird game console peripherals out there, and it's difficult just picking one thing, but I'm sure having more to look at and read about isn't bad hehe. I took a photo of my Game Boy Printer, Nintendo e-Reader, SNES Voicer-kun from Koei, and Turbo File Twin from ASCII.
The most
fun tech out of these four is definitely the printer; the stuff you can print from licensed games is fun enough (like all the silly printable things in Super Mario Bros. Deluxe) but the fun really elevated when I wrote
a Game Boy program that lets you scroll through an album of pictures and print them, and I have had way too much fun at family parties whipping out my Game Boy and printing all sorts of silly stuff. There's just something inherently funny about watching it slowly spit out the ALDI logo onto thermal paper.
The e-Reader is neat. It's really cool to me to have games stored on paper and have that actually work. I have a bunch of Animal Crossing cards and each one has a unique letter displayed on the GBA when you scan it, and a second unique letter when you scan it into the GameCube game, and these letters add a lot of per-villager characterization beyond just the personality types.
Going into more obscure, Voicer-Kun is an infrared transmitter/receiver that plugs into a SNES controller port, and the purpose is to control CD players in order to have CD audio accompanying games. We never got the SNES CD add-on so this happened instead. I haven't played any of the games that released that use this (and I don't have an infrared controlled CD player or know Japanese - which is very important because I think the supported games are visual novels) and I got it because I thought it would be really fun to write code to have my SNES interact with a TV remote control, or a Tamagotchi Connection, or a Game Boy Color. I never ended up doing that though.
Turbo File Twin is effectively a memory card for the SNES, following a similar product that was a memory card for the Famicom. Including a save feature in a cartridge was not free and this was a cost cutting measure, but also meant that you could have the possibility of multiple games interacting with the same save file, even if they're on different consoles (there was an adapter to plug the Famicom version into a Super Famicom) - supposedly the
Wizardry series lets you bring characters between games. The Japanese versions of RPG Maker on the Super Famicom support this peripheral, which sounds like the kind of game where you really would want to expand the storage past what's in the cart.
I guess my Super Famicom accessories are more just fun to look at and think about the possibilities and my printer and e-Reader are fun
right now. I've seen people making new e-Reader games (and I ordered some recently) and I hope that becomes a bigger thing.