ZigBee is a wireless standard that uses the same 2.4GHz radio band as Wi-Fi and Bluetooth. It offers less bandwidth than Bluetooth but has a range of up to 30 metres (10 meters for Bluetooth) and since it is less power-hungry, Zigbee suits devices that transmit data non-stop.

Motorola presented its first consumer application at NextFest in San Francisco:
a key hanger that wirelessly connects to a mobile phone, warns the owner when the two get out of range and tell him if it is at home, the office or some outside location.

Similar solutions can be built into other easy-to-lose items.

Read vunet.