Slightly branching out from the usual western handhelds, I've seen WonderSwans but never played around. Then managed to pick up a faulty "Color" model for £40 on eBay listed as not working.
On inspection it looked, internally, very clean - no corrosion or damage. From my experience with Pokémon Mini handhelds, I was wondering if perhaps the power button wasn't getting the signal.
So I applied a slice of kitchen tin foil, some sticky tape and voilà, it powers up! I had to also apply it to the sound and start button (and I'll probably make this permanent with conductive paint one day), but otherwise it's all good.
(And I snagged a flash cart so I was able to experience some translated romhacks too)