XRay is a mashup of Microsoft Surface and the iPhone. Stimulant created a Surface application that reveals photographs, but by laying an iPhone on Surface, more layers of imagery are revealed, synced in real time with the upper image layers shown on Surface. The only limit to how many iPhones can be used at once is, well, how many iPhones you have handy.
If you're wondering, this works on Windows Mobile and Android phones as well. Stimulant was inspired by the Microsoft Research project that demonstrated "Second Light," a new variation of the company's Surface technology, at PDC 2008. For sake of reference, I've also embedded the video of that technology below:
Source:→ Ars Technica