A neat-looking device for DS homebrew development is natrium42’s DSerial. The features include

• 8051 microcontroller running at 24MHz
• Reprogrammable from DS, premade firmwares available
• Free development tools available
• 18 GPIO lines, 2 status LEDs
• UART with RS-232 level converter (can be disabled)
• Full-speed USB 2.0 device (does not support USB host)
• PWM and ADC available
• 2D tilt sensor

I’m hoping to make some cool stuff with it…my first plan is to build a portable heart monitor, that can also tell you how stressed-out you are, based on your ‘coherence’ (essentially you take the FFT of your heart rate over time, and you want most of the energy to fall near to 0.1Hz)