A Sagan Institute Box.
A Sagan Institute Box is a box constructed by the Sagan Institute, a noted American institution named after Carl Sagan, designed for the occasion that Earth could somehow, hypothetically, get the box to a distant world.
The box contains information pertaining to the people of Earth, extending a hand of friendship. As an additional feature, Captain Samantha Carter installed Iridium in the boxes. The distinct nuclear signature of iridium could be detected in the event that a Sagan Box impacted the iris, thus enabling the SGC to determine if it was safe to open the iris or follow-up off-world. It became standard procedure for SG teams to give Sagan Institute Boxes to the peoples they encountered if first contact went well.
Dr. Daniel Jackson gave a box to Kendra in hopes that Thor and his people will return to take it. On Kendra's instructions, Gairwyn later threw the box through the Cimmerian Stargate, in the hopes of receiving SG-1's aid against Heru'ur. (SG1: "Thor's Hammer", "Thor's Chariot")
A Sagan Institute Box was given to the Tok'ra after they tentatively agreed to work with the SGC. It was later used by Martouf so that he could travel to Earth to warn them of Sokar. (SG1: "The Tok'ra, Part 1", "Serpent's Song")
As the SGC built stronger and more formal alliances with off world allies such as the Tok'ra and the Rebel Jaffa, key individuals such as Bra'tac were given personalized GDOs and IDCs. Sagan Institute Boxes were thus reserved for cases of first contact with less technologically sophisticated groups.