Yep, the wonderswan. The only real alternative to the GBA, released only in Japan. I don't speak Japanese (yet) but I bought one (WSColor) all the same, and I bought Guilty Gear, Mr.Driller and Gunpey EX.
I think that it's a really nice little gadget, because it only cost me £20, and will run for 4 hours on a single AA.
Does anyone else have one of these, or is it just me?