The best GPS tracker is, of course, the iPhone (and probably other smartphones, but somebody else needs to fill us in on that - I haven't got a clue).
One of the apps for this is called Friend Tracker ($1.99), but there are others.
The Friend Tracker allows other authorised phones to see where you are on the map.
And as a person that has lost her iPhone this afternoon (I suspect it is lying in the kiddie's classroom, but I cannot be sure till tomorrow morning) -
EVERYBODY SHOULD HAVE THIS ON THEIR IPHONE SO THAT THEY CAN TRACK IT WHEN IT IS LOST.
I didn't. If I find my beloved phone, I WILL put it on.