Technology designed to tag targets for Asgard transporter beams to gain a lock. Objects can differ from a human to a target as massive as a Stargate. The small device can be easily concealed, and does not need to be activated remotely. When always transmitting the coordinates the beacon gives off a minute energy anomaly, which can easily be fooled for other things.
The larger transponder device was succeeded by the smaller locator beacon, and eventually S.G.C. personnel were given subcutaneous implants to allow them to be tracked and beamed out of hostile territory.
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.