Using an AVR microcontroller pretending to be a Guitar Hero controller for the Wii, connected to the Wiimote's I2C bus with a FireWire cable. Buttons are actually controlled by my keyboard (sent to the AVR via serial port).
This took a while to make because of the data encryption between the guitar and the Wiimote. Thanks to Hector Martin for figuring out the encryption!
I'll have more details when I finish a bit more (still some problems with encryption). My main goal is to make a drum set for Rock Band 2 and Guitar Hero World Tour.