Milwaukee Servoelectric Guitar is an ingenious experimental instrument. Individual servos for each string control the string tension, with a clever compensator wheel arrangement. The servos are powered by low cost home made amplifiers. The tension / frequency sensing circuit is a novel design incorporating a potentiometer and springs. Solenoids to pick and damp the strings would fully automate this instrument.
This projects shows how to make a 3 string slide guitar from a single birch board 1x3x30 inches, and bits of scrap. The good thing about this guitar is that there are no frets to worry about, no truss rod, in fact it is really just a plank with a bridge, pickup and some tuners. You can make markers where the frets would go, to help you get your bearings when you are playing it. To save cash you could use a glass bottle for the slide (like in the picture).
Guitar robots anyone? Take a look at GuitarBot – built as part of the League of Electronic Musical Urban Robots (LEMUR) project by Eric Singer, Kevin Larke and David Bianciardi. The aim of the project was to create an electric slide guitar, with all the dynamics capabilities of a human player but with the potential to exceed the physical limitations of any human player.