I got this from Gekobot's blog, but it's so cool I thought it deserved a repost. It's a demonstration of "Augmented reality" on the 3DS.
"Augmented reality (AR) is related to a more general concept called mediated reality, in which a view of reality is modified (possibly even diminished rather than augmented) by a computer. As a result, the technology functions by enhancing one’s current perception of reality. By contrast, virtual reality replaces the real-world with a simulated one."- from Wikipedia.
This got me really excited, I haven't owned a hand-held since the Gameboy Pocket, but this has made me consider upgrading my crusty old GB. Especially with a Legend of Zelda: Ocarina of Time remake being one of the launch titles.
If you liked that then please check out Gekobot's blog, he has a lot of cool posts like this one.