Alternate reality game
From Halopedia, the Halo wiki
An alternate reality game is an interactive narrative that uses the real world as a platform, often involving multiple media and game elements, to tell a story that may be affected by participants' ideas or actions. Halo has had three ARGs: i love bees, Iris, and Hunt the Signal.
I Love Bees
During the summer of 2004, the website ilovebees.com was used as a publicity site for Halo 2, with the site being pointed to by adverts for the game during movie trailers. Ostensibly a beekeeper's personal site, the server appeared to have been taken over by an unstable A.I. whose thoughts are scattered on the site.
The front page had a counter counting down to July 27, 2004 (when it says "network throttling will erode"), August 10, 2004 (when "this medium will metastasize"), and August 24, 2004 (at 8:06 am, when it will be "wide awake and physical") - many thought something big would happen related to Halo 2 on these dates, however it was only related to the ILB game. Currently there is a counter counting down to the day the Covenant will attack Earth, over 500 years in the future.
Iris was a Halo 3 viral marketing and public relations campaign. It was designed to give people more information on how the great conflict behind the Halo franchise all began, as it builds up to the climactic conclusion of the series on September 25, 2007, the release date of Halo 3.
The campaign ran for several weeks throughout the summer and promised to be a compelling experience to uncover valuable information. Users who helped to unlock key information were awarded with t-shirts and other small prizes. Unfortunately, Microsoft would not say whether there would be any other rewards for participating.
Section 3 ARG
The campaign lasted for two years, giving fans a look into the Office of Naval Intelligence's Section Three and also insight into the experiments they did. Users found glyphs that led to codes being discovered for use on Halo Waypoint (now only on the Xbox 360). While the users found codes, an AI that was originally meant to help them turned against them, giving the puzzles for the ARG.
Hunt the Signal
Hunt the Signal was a competition related to the Hunt the Truth marketing campaign for Halo 5: Guardians. During this competition, which was open from May 5 to May 22, 2015, teams of four competed to solve several puzzles and challenges. The winning team would have characters in Halo 5: Guardians made in their likeness; the first team to complete the competition did so on May 16, 2015.
This competition was promoted on YouTube and in movie theaters during a Ford F-150 advertisement, as well as at the end of episode six of Hunt the Truth. Canonically, the contest served as the efforts of an Unidentified AI tasked with probing weaknesses in Spartan-IIs and their MJOLNIR Mark VI armor by Jameson Locke as per STOLEN GAUNTLET protocol during the latter's search for John-117 and Blue Team. However, cut content that was discovered during the contest suggest that the ARG was originally meant to promote Halo: Reach, relating to Dr. Halsey's work on a Forerunner artifact and Cortana, but had been abandoned and latter re-purposed for Halo 5: Guardians.