Charmed Labs manufactures an interesting Gameboy-based system that transforms a Gameboy into an embedded controller that can be used, among other things, to make Lego-based robotic systems. This product page for the Xport Robot Controller includes a number of demonstration videos in WMV format that are pretty interesting. Cost is $140 to $200 depending on the just how much you want in the devkit — Gameboy or Gameboy Advance not included.
Rhinoskin make a couple of awesome-looking cases for the GameBoy Advance SP. Only $15, and available in either platinum,
Creatures Labs is porting their artifical life game Creatures to the Gameboy Advance.