Well Now I know. From Wikipedia:

Communication device with stones

A dome-shaped device with a blue crystal spire designed to facilitate communicate across vast distances. The device works in tandem with palm-sized oval stones. When inserted into one of twelve slots on the base, the stones allow the user's mind to take over the body of whoever last touched the stones on the other end, provided someone with the ATA gene activated them.[21] The reverse is also true if those on the receiving end make use of a similar device. The device draws its power from subspace (the same way a ZPM does), and works seamlessly even when communicating over billions of light-years apart.

In addition to their regular function, two communications stones can form a psychic link between the last two people that used them. SG-1 retrieved one of these stones from the same planet where the quantum mirror was discovered. The stone was kept in Daniel Jackson's lab and inadvertently activated by Jack O'Neill. Another stone was purchased at a yard sale by Joe Spencer, a barber from Indiana, who was then able to see O'Neill's memories as he recalled them while writing his mission reports.[65]

One of these devices was discovered under Glastonbury Tor, and it was subsequently used by Dr. Jackson and Vala. It transported their minds to the Ori galaxy into the bodies of a married couple, Harrid and Sallis.[21] Their minds were sent back by a similar device, but the transfer was cut short by a Prior. Col. Mitchell and Teal'c eventually destroyed it by throwing it into the Stargate's unstable vortex, bringing Daniel and Vala back.[66] Several months later, Vala Mal Doran contacted the SGC from the Ori Galaxy using the device thanks to the Ori underground resistance, and warned SG-1 about the nearing Ori invasion of the Milky Way. However, the link was cut by her husband Tomin.[67] Another communication device was found in the Pegasus galaxy, switching Dr. Keller's mind with that of a thief. Both Keller and the thief were able to use each other's bodies despite the lack of a second device. Unable to find any shut-down switch, Ronon shoots it to break the link.

In Stargate Universe, Stargate Command has apparently devised a more compact version of the communication device. This version is a boxed-shaped device with room for five stones in an 'X' formation. The stones are simply placed on the flat panel of the device to activate it. The team aboard the Destiny has access to one, as does Stargate Command. Dr. Nicholas Rush uses it to take over the body of Dr. Bill Lee at the SGC, who had been waiting by the device. Later on, Col. Young and Chloe Armstrong use it to trade places with Col. Telford and a doctor, respectively, demonstrating that the transfer works both ways.