The Cell Key is a rare item found and used in Lunpa Fortress in Golden Sun. In the room where it is visible and out of reach, if you are facing down towards it from behind a locked door, use the Catch Psynergy to retrieve it and all green doors in the dungeon will open while it is in your inventory.
This item can be sold, and since it counts as an Artifact, it can be bought back again as well. One door in Lunpa Fortress is often missed by players because they pass by it before getting to the room with the Cell Key. The door leads to a room containing the Mythril Circlet, one of the game's best items.