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: