Don't get too excited, as this isn't a commercial product yet, but Nokia have shown off Locate Sensor, a tiny gadget that can be attached to anything precious and then which communicates with your S60 phone. If the aforementioned gadget then gets separated from you, you can track it down (direction and distance) up to 100 metres away. So perfect for keys, laptop, etc. There's a reverse mode, too, which can act as an anti-theft device. See Electric Pig for more info and photos. 

Via Nokia Conversations