I Love Bees, the alternate reality game (ARG) which served as a viral marketing campaign for Halo 2 had me and Allison running around Portland waiting for pay phones to ring. Very fun and very cool.
Today, it seems another ARG may be in the works; this one for Halo 3. According to Joystiq some sort of artificial intelligence calling itself, Adjutant Reflex, has infiltrated the Bungie forums and has been posting mysterious messages.
I'm currently sorting through all of the various information that is out there and will update you if I hear and/or discover anything.
Hat tip to Joystiq