One downside of working at PARC is that a lot of technologies sound so out there or address very specific (fundamental) problems that they don't make for good cocktail tales.
Responsive mirror, on the other hand, uses cool technology in a context that we can all relate to. It lets you do side by side real-time comparisons of clothing with prior pieces as you put on a new one instead of conjuring up the images of what the prior piece look like.
Of course, this is not one of 'em before/after pictures in diet pill infomercials. Responsive mirror follows your actual movement and plays back the same movement with the prior article. In other words, you can do not just "apple-to-apple" comparison but "same-side-of-apple-to-same-side-of-apple" comparisons. Now, that is cool!
Here is a link on the tool: http://blogs.zdnet.com/BTL/?p=21045
Of course, under the hood, PARC has built up a platform on a robust video analysis and given the proliferation of both cameras and screens beyond laptop computers (think smart phones), there are a lot of applications that are still waiting to be imagined.