My friends and I like to coordinate for our mutual friends' weddings. Our friends L & E got married recently and we wanted to do something special to mark the occasion. We settled on something elegant and simple -- pocket squares that could visualize the number of people within a certain proximity. For each pocket square I sewed 4 LEDs to a LilyPad Arduino and programmed it to display the number of other pocket-square-wearers present, using the XBee's RSSI data for coarse grain distance approximation.
Via and Source: Instructables