Xport 2 Turns Game Boy Advance Into Embedded Microcontroller
Thanks to Gizmodo for its article discussing the Xport 2.0 unofficial cartridge add-on for the Game Boy Advance, which allows users to turn their GBA "into a user-programmable embedded microcontroller for directing robots, capturing images or video, or operating anything else you can interface to its user-programmable I/O." There's more information on the official Charmed Labs site.
There's a free GBA Programming Book available in .pdf format online at http://www.jharbour.com/
that's worth taking a look at if you're intersted in GBA Programming in general.
I'm tempted to try out this kit to see how it fares...$150 is a bit steep of a price for me right now though.