If it weren't for the latest issue of the excellent Retro Gamer magazine I would never have looked into the rather obscure handheld console that is Nintendo's Pokemon Mini, and that dear friends would have been quite a shame. This handheld is after all the tiniest one Nintendo ever released and, despite looking like some sort of LCD game, by far the smallest machine that works with cartridges I've ever seen. Pokemon Mini sports a small black & white screen, an accelerometer (!), a vibrator force-feedback thingy, a real-time clock and infrared connectivity. 10 official games and a ton of homebrew ones have are already available for it and you can find out more about it and even try an emulator via the excellent Pokemon-mini.net.
Searching around eBay, I did find this lovely, though not exactly dirt-cheap, Pokemon Mini. Seller ships only within the US and is offering the Wooper Blue version of the system, complete with one game (Pokemon Party Mini). Shockingly, this is the cheapest one I managed to find, as prices for a console with a couple of games can go up to 100$; possibly more.