Guitar Hero has spawned a large number of mods, hacks and general tweakery, with gamers often looking to use proper instruments in place of the normal controllers. However this is the first time I've seen a Guitar Hero controller converted in the opposite direction; it now works as a MIDI guitar, compatible with any synth with the necessary port. Best of all, there's actually no damage to the controller's original functionality, so you can switch between gaming and music making to your heart's content.
As mods go, this is a thorough one. The guy behind it, Dave, started out by writing a user manual for the whole thing, and he has made that - together with instructions and a guide to the MIDI functionality - available for anyone to recreate the project. All the controls work, including the whammy bar, and there's control over the scale being played, the tonal modes, timbre and resonance, and even single-touch chords in a preset key.
What'd be a neat addition would be wireless MIDI - then, as Chris Scott Barr might say, you could truly rock out. In the end it cost around $105 to create, including buying the guitar controller itself.