A removable, U-shaped unit worn directly in front of the left temple, capable of receiving information directly from The Link and transmitting that data in a comprehensible format into the user's brain. The Link connected the inhabitants of P3X-289 to a super-computer, which was able to alter their memories via the interface.
When SG-1 arrived the local townspeople offered their Link nodes to allow the team access to information about their people -- but SG-1 cautiously refused. It wasn't long before the Link transmitted false information to the minds of the inhabitants, telling them that removing the Link node would be fatal.
Revisions - The people of P3X-289 nearly force link nodes on Jack O'Neill and Teal'c when the super-computer determines them to be a threat to the status quo it is programmed to maintain.