Charmed Labs has come out with a homebrew GameBoy Advance insertable called the Xport 2.0. The naked widget plugs into the cartridge slot of the Game Boy Advance and:

[...] allows the Xport hardware to be tailored to your target application, and the 64 digital I/O signals provide programmable I/O for interfacing to practically any hardware device.

The website has some awesome .WMVs of the GBA-bots in action, demonstrating basic wandering and barrier-avoidance (complete with event-triggered They Might Be Giants audio samples from the bot itself), holonomic motion (frisbee-like spinning while scooting across the floor), and a video entitled "Teach" in which the GameBot is shoved around like a toy car being played with, and then released. It then autonomously repeats the movements it was "taught" just a few seconds ago. Outstanding! Eliza Gauger

Gameboy + LEGO = Robot [Charmed Labs]