|Smart Helmet with built in Arduino|
I think that this is a really useful gadget which I would use (unfortunately he is not in a position to accept advance order yet), but what really impressed me was how such a young student was clearly very well informed about the technology scene. When he found out that I knew about Arduino devices he immediately launched into a discussion about the limitations of the USB interface on the board (the quick summary is that it doesn't quite match the formal interface specification). He also gave me pointed to some web sites for buying components and advised me on how to tackle a project I am planning with my recently acquired weather station.
DISCLAIMER: I did not get a chance to see all of the projects (or even a small proportion of them) so there may be many better projects - I will await the results of the formal judging which will be announced this evening.