Not directly relevant to real-time rendering (although there might be some tangentially related applications in areas like augmented reality), this SIGGRAPH 2009 paper is just painfully clever.  It exploits the phenomenon of bokeh (the large circular blobs that small intense light sources generate in out-of-focus images) to create tiny barcodes that can be seen from a distance by cameras.  They put a lenslet over the barcode, so that when viewed in a defocused manner you see a large circular blob – with a sharp image of the barcode in the center!