There has been talk for some years about Japanese mobile phones that would alert the user in some way when another similarly set up phone was in the area. The idea being that you and your friends can use the same code, and get a quick notification when someone you know turns up.

The apparel millionaires at Think Geek have taken the idea a step further with the proximity-detecting t-shirt. When two of these shirts are in the vicinity of each other, the hearts on the front of each shirt light up, so you can easily tell when your girlfriend or boyfriend is around (assuming he or she is wearing the shirt, of course).