Look at this amazing DIY live audio sequencer designed at University of Southampton, only using a webcam, a few print layouts and a pc/mac they created a tangible user interface. In their words:
«d-touch is a visual marker recognition system that enables the development of low-cost tangible user interfaces and mixed reality applications. d-touch supports markers that areboth machine-readable and visually communicative to humans.»
Check out their website for more info and the DIY instructions!