Everybody loves lasers, right? That's why I decided to build a "laser harp"-style midi controller during my time studying Sound and Music Production at h_da darmstadt. This project is the first one ever where I etched a custom PCB. Listen to a short radio report about it here (Radio FFH, german):
The laser beams are used control effects in traktor DJ Software in realtime (for example stutter, reverse or delay). It is obviously also possible to control any midi-enabled instrument or plugin with it.
It consists of an arduino, twelve laserpointers and some electronic parts to control the on/off treshold of 12 photosensors. If a laserbeam is interrupted it does not shine on the sensor and a midi note is triggered. The communication with the instrument/computer is done via midi, which means it is also possible to use a hardware synth together with the controller without a computer in between. The materials needed are actually quite cheap. (I suppose as of today the whole build can be done for around 40€)