The Secret Intelligence Division, or the Secret Intelligence Agency, is the Freedom Fighters' primary intelligence gathering division.
Its operatives placed in a sophisticated network among Dr. Ivo Robotnik's territories, collecting information on his current plans and operations and spreading it to the rebel groups. It's primary contact was the Knothole Freedom Fighters, and it's leader was Director Harvey Who, who, in order to maintain cover (Robotnik made attempts to expose him in the past), went out on field assignments despite the dangers. (StH: #16)
Following Robotnik's death at the hands of Sonic the Hedgehog, re-formation of the Royal Secret Service and open war being declared with the Eggman Empire, the SID has presumably been dismantled. (StH: #50, #61, #130)