Slapyak has an interesting tutorial on creating a Guitar Hero MIDI controller.
With the current version you can:
play 2 octaves of an 8 note scale at a time
change the starting note anywhere in the range of a regular keyboard
change octaves
change keys
change tonal modes (Ionian, Mixolydian, Lydian, Dorian, Aeolian, Phrygian, Locrian)
change the timbre of a sound
change the resonance of a sound
use the whammy bar to pitchbend
play chords in the mode you have the guitar set to (major/minor/etc.)
rock out with a video game toy
Here’s a high-level flow for the MIDI controller:
Guitar ?> Arduino ?> LEDs & MIDI ?> external MIDI device.
Details are available at the Slapyak site.
Here’s a short video of the Guitar Hero MIDI controller in action: