From Halopedia, the Halo wiki
Believe is a campaign focus on several connected in-universe events and story elements. This includes a historic recollections of the Second Battle of Mombasa in 2610, the introduction to the Museum of Humanity and the story of Jake Courage a long with many more smaller elements. The old Believe website, while now defunct, is now available to view in its entirety thanks to the Halopedia archives, and can be viewed here (Flash player required).
IRIS was a forum based alternate reality game for Halo 3. IRIS details both the story of the Forerunners, fighting against the Flood as well as the Society of the Ancients. The Halopedia archive maintains archives of the original IRIS website, which can be viewed here (Flash player required).
The Halo 3 announcement trailer was a trailer shown off at E3 2006. It features Master Chief on Earth as the Covenant flyby, along with the ghostly apparitions of Cortana.
The Halo 3 E3 2007 Trailer was a trailer shown off at E3 2007. It features numerous gameplay and cutscene footage from the campaign of Halo 3.
Starry Night is a short, mixed live action/CGI film about the Master Chief's childhood and as well as one battle on Earth as an adult soldier.
Darkness is a short trailer for the Legendary Map Pack that is primarily composed of gameplay footage of the three maps in the pack along with a reading of a poem written by George Byron.