Halo 3 Terminals
As per the hint given by Terminal.jpg from Server 05, Halo 3 contains 7 terminals scattered throughout the Forerunner structures. While not necessary to complete the game, finding all 7 unlocks the Marathon Man achievement worth 40G.
These terminals also give more clarity and continuation to the backstory that began in Iris about the Forerunner firing of the rings 100,000 years ago.
Below are listed each Terminal, location, and data within each. Note that these pages do contain spoilers if you wish to find them yourself. Proceed with caution!