Guiboard app for iPhone and iPad
Use your Guitar as a MIDI Keyboard!
Guiboard captures the pitch of the incoming audio, and converts it into MIDI notes. The recognized MIDI notes are displayed with musical notes, guitar fingering, piano fingering, and played with internal test sound. Also with support for CoreMIDI, Guiboard can send MIDI output to the other app, or to the connected device through cable or WI-FI.
If you connect your guitar via a guitar interface cable, Guiboard work better. Supported guitar interface cables are IK Multimedia iRig, Peavey Ampkit Link, Apogee Jam, Sonoma Wire Works GuitarJack, etc.