An obelisk of Ancient design. The Ancient text on the outside defines the object as an aperture, or gateway, to "enlightenment." In reality the device is capable of sending an individual kilometers across the surface of a planet -- on the Sodan world, bypassing the Mountains of Dor Tanak, to the village on the opposite side. It can even transmit an object's energy pattern through an active Stargate to a receiving transporter on the other end.
The transporter has Ancient writing on the external surface. Its inner mechanisms are highly complex, accessible via a control interface that slides out from the side of the column.
What is not known is whether or not the device itself is responsible for initializing a personal gateway, though Samantha Carter did detect an energy signature within the column of stone. It is also not known precisely how the device itself functions. But it seems likely that there must be an identical column on the receiving end.
Morgan Le Fay used the technology in a extraordinarily unconventional way, positioning transporters next to Stargates on many worlds and programming them to automatically transport an entire cave and its inhabitants (including Merlin, who she left there in stasis) through the Stargate and on to another world in her planetary circuit. This was programmed to occur at regular intervals -- keeping the cave and its secrets on the move and extremely difficult to breach.
Babylon - SG-22 locates an Ancient transporter, the device responsible for allowing the Sodan to flee into safety so quickly.
Arthur's Mantle - A transformed Volnek steals a crystal from the Sodan side of an their Ancient transporter, preventing anyone from using it to escape the area.
The Quest, Part 2 - SG-1 and Baal are trapped with Merlin in Morgan's planetary circuit, which uses Ancient transporters to send them through the Stargate from one world to the next.