"Place Lab is software providing low-cost, easy-to-use device positioning for location-enhanced computing applications. Place Lab tries to provide positioning which works worldwide, both indoors and out (unlike GPS which only works well outside). Place Lab clients can determine their location privately without constant interaction with a central service (unlike badge tracking or mobile phone location services where the service owns your location information).
The Place Lab approach is to allow devices like notebooks, PDAs and cell phones to locate themselves by listening for radio beacons such as 802.11 access points, GSM cell phone towers, and fixed Bluetooth devices that already exist in large numbers around us in the environment. These beacons all have unique or semi-unique IDs, for example, a MAC address. Clients compute their own location by hearing one or more IDs, looking up the associated beacons' positions in a locally cached map, and estimating their own position referenced to the beacons' positions. "