There are quite a few Rock Band guitar controllers on the market, ranging from the cheap to the surprisingly expensive, but one thing they generally have in common is the plastic strum bar. It doesn't have to be that way, though; one enterprising gamer has swapped out the plastic for a beam of infrared light which is recognized as a strum whenever you break it.
Video demo after the cut
The beam itself is around 5mm wide and will respond to traditional strums but also to tapping, letting you play much faster (though that's really cheating). Since there's only one beam, though, you can only have an upstrum; an interesting future mod could pair two more directional lights, such as lasers rather than IR, and work out up- or downstrum via the order in which they're broken.
Still, it looks relatively straightforward to do, and gameplay is apparently faster and now allows you to "really ham it up" with extra-long guitar strokes. Ironically it makes Rock Band even more like playing air guitar.