Xbox.com is the official Microsoft website for the Xbox and Xbox 360 consoles, as well as the online gaming service Xbox Live. It has pages detailing popular Xbox games, including each of the Halo games.
Of note is the backstory on the Halo: Combat Evolved page, which was the first official source to the events that led to the formation of the UNSC as well as human colonization. However, with the creation of the new Halo official site, the original Halo: Combat Evolved page has been taken down. The timeline is still hosted by Halo.bungie.org at their Halo Story Page.