Technology designed to tag targets for Asgard transporter beams to gain a lock. Objects tagged by the beacon can differ, from a human to someting as massive as a Stargate. The small device can be easily concealed, and does not need to be activated remotely -- giving it a slight advantage over its larger predecessor, the emergency transponder. When always transmitting the coordinates the beacon gives off a minute energy anomaly, which can easily be fooled for other things.
For the safety and security of Earth's off-world personnel, the larger beacon device was quickly replaced by a subcutaneous implant.
Endgame - The Trust use locator beacons to move the Stargate to their new Al'kesh, and themselves from the surface to their ship.
Ethon - S.G.C. tags Dr. Jackson with a locator beacon before sending him back to Tegalus with Jarrod Kane, but the beacon is found and separated from his body.
Dominion - Baal sends his Jaffa to take Adria from SG-1's hands, using a locator armband to transport her to his ship.