Ancient repository of knowledge
Device of Ancient origin, capable of "downloading" vast amounts of information on their language, history, and technology into a user's brain. The repository, when confronted with a willing (or unsuspecting) individual, is able to discern whether or not the subject is a Goa'uld or Jaffa. If not, it locks the head of the individual in place, allowing the data to be transmitted into the brain by means of what appears to be a stream of high-intensity light.
The library of knowledge begins to gradually unspool in the recipient's mind, eventually replacing his or her spoken language with that of the Ancients. Information including instruction manuals behind the Ancient technology overruns the mind, and after several days the person becomes unable to communicate with others -- though he or she can increasingly take advantage of the knowledge of the Ancients. The repository contains plans detailing instructions on how to repair a damaged D.H.D., upload Stargate addresses (including for many worlds unknown to the Goa'uld) into Earth computers, build power-generating devices, and create a weapon to combat the Replicators.
As a former ally of the Ancients, the Asgard have the ability to remove the Ancient's knowledge from a human brain. When Jack O'Neill sought their help in doing so, they explained that the technology was not meant for humans -- and were surprised that he was even advanced enough to use the knowledge in his head to find them.
The Fifth Race - Colonel O'Neill looks into the Ancient repository and inadvertently downloads their entire archive into his brain. His subconscious mind eventually gives him information that allows him to travel to the Asgard world Othalla to get help.
Lost City, Part 1 - O'Neill interfaces with another repository in hopes that the information he learns will lead the team to find the lost city of the Ancients. The device is destroyed before it can fall into the hands of Anubis.
The Quest, Part 2 - Merlin uses a custom repository inside his secret laboratory to download his own knowledge and consciousness, transferring it into Daniel Jackson's body. His device also serves as a control interface for his weapon assembler.
The Shroud - Daniel uses Merlin's assembler interface to complete the anti-Ori weapon.