DS Brut is an open source hardware prototyping platform for the Nintendo DS, aimed at enthusiasts, artists and game developers alike. We manufactured Slot-1 cartridges that allow you to control a basic UART, up to six GPIO lines, up to three ports for PWM, up to two ADCs, and I2C bus – all from within your own DS homebrew application.

The cartridge features one Atmega168 microcontroller which – like the Arduino – can be very easily programmed via the TX/RX pins. By default, the microcontroller runs a programm (source code available) tailored for use with a LGPL-licensed library for devkitPro, that makes it possible to access the microcontroller’s peripherals from within your DS code. The library transparently handles all communication with the cartridge over SPI bus, which makes it extremely easy to use for developers. As its API closely resembles those of Arduino (analogRead(), digitalRead()), you can start prototyping right away if you have some experience on that platform!

Discussion: http://forum.gbadev.org/viewtopic.php?t=16160