You can turn your guitar into into a MIDI controller with the MIDI Guitar app on the Mac App Store.
We’ve tried it (plugged our electric into our audio interface) and the results were passable, but we don’t think technology is quite where it needs to be for this to be perfect. We had to play slowly to have it register all the notes, and it did feel a bit latent compared to a regular keyboard controller or drum pads. However, the video we saw seems to have better results. Our guitar skills may not be quite what the dude in the video’s are.
Still, it’s available for free (or $40 to remote the ads), so it’s worth testing out.
For those who don’t know, a MIDI controller sends note information to a synth or computer to play it back with any instrument you choose. This can make your guitar sound like a dirty FM synth, or thick subtractive synth. In the end, you are not hearing the actual guitar, the note or chord is being analyzed and converted into MIDI notes and what you hear is your synth or effect of choice played through the speakers.