Controlling music playback devices with MIDI is becoming quite commonplace, however modifying existing instruments to be computer-controlled is an interesting challenge and one that you can do yourself. One excellent example of this is by Mike Cook, who took a simple childrens' Glockenspiel - added solenoids under the bars and wired the whole thing up to an Arduino with a MIDI interface. Now this can be controlled using almost any PC with a MIDI output capability. Here's Mike's work in action:
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