The one pinned is very, very out of date, so I'm creating this. Just a list of sources of info, reliablity and time period.
Also, Catalog's Waypoint posts are to be taken as canon at the time they are posted. Similarly, things like the eleventh hour reports, HunttheTruth, IRIS, ilovebees,data drops and Canon Fodder should be taken as canon. Because these smaller sources are too numerous to list here, this page only covers released media, though a link to the Halo Archive's "Absolute Record"- a series of transcripts and links of these sources- will be provided.
In addition, GrimBrotherOne of 343i confirmed that all visuals and sounds of an Anniversary remake should be taken as canon over the original, one example being the Halo 2 (Anniversary) Gravemind. Similarly, it is now confirmed that the multiplayers of every Halo game take place within the War Games simulations and that every map is also recreated by UNSC technicians for training exercises.
Oh yeah, this list only covers purchasable material. Books, games, comics, DVDs and so forth. Aside from the ARG stuff and trailers. So the smaller tidbits aren't included, only things you can buy.
Halo: Combat Evolved (Anniversary) 2552. Concurrent with Halo: The Flood
Halo 2 (Anniversary) 2552
Halo 3- 2552/Early 2553
Halo 3: ODST- 2552 (Concurrent with Halo 2)
Halo: Reach- 2552, partially concurrent with Halo: The Fall of Reach
Halo 4- 2557
Halo 4: Spartan Ops- 2558.
Halo: Spartan Assault- 2554 (Simulation), 255X (Out of simulation)
Halo Online- postwar period (2553+)
Halo: Spartan Strike: Rotates between 2552 (Mombasa) and 2557 (Gamma Halo) (Simulations), 255X (Out of simulations)
Halo 5: Guardians- 2558
Halo: Cryptum- Circa 100,000 BCE
Halo: Primordium- Circa 100,000 BCE
Halo: Silentium- Circa 100,000 BCE
Halo: Rebirth (joint with Silentium)
Halo: The Fall of Reach- 2517-2552. Partially concurrent with Halo: Reach.
Halo: The Flood. 2552 Concurrent with Halo: Combat Evolved.
Halo: First Strike. 2552.
Halo: Ghosts of Onyx- 2552
Halo: Contact Harvest- 2525
Halo: The Cole Protocol- Early in the human-covenant war
Kilo-5 Trilogy (Glasslands, The Thursday War and Mortal Dictata)- 2553
Halo: Broken Circle- Early Covenant Era- 2552
Halo: Hunters in the Dark- 2555
Halo: Last Light - Late 2553 (After the Travissty)
Halo: New Blood (Various. Narration set between 2555 and 2558)
eBook Novellas/ Short Stories
Halo: Evolutions I and II (Not an eBook, but a collection of short stories)
Halo: Saint's Testimony - 2558
Halo: Shadow of Intent - 2553
Origins (I+II)- 2554 (Forward Unto Dawn), various time periods (Story)
The Duel- Early Covenant era
Odd One Out- Non-Canon. 25??
The Babysitter- 25??
The Package- 254?
(Note that dev. commentary on Halo Legends confirms that all appearances of Doctor Halsey aren't necissarily canon, but rather art style.)
Forward Unto Dawn- 2526
Halo: Nightfall- 2556
Halo: Landfall- 2552.
Deliver Hope- 25??
We Are ODST- 25??
Other live trailers- 25-various
Halo: The Butchery of Reach Animated - it sucks don't watch it
Comics (Note that, once again, art style cannot always be canon due to artists interpretation)
Blood Line- 2552
Uprising - 2552
Graphic Novel - 2552
Initiation- 2553 (At least post-Reach, due to Jun being alive, before Spartan Assault)
Encyclopedia- (Two versions)
Halo 3 EVG
Halo 4 EVG
The Art of Halo 3
The Art of Halo 4
The Art of Halo 5: Guardians
Halo: Combat Evolved Anniversary- 100,000 BCE-2552
Halo 2: Anniversary- all story arcs take place in 2552, the story of the arbiter takes place over the lifespan of the Covenant
Halo 3- Pre-Halo Firing
Halo 4- Pre-Halo Firing
Halo 3: ODST Audio Logs- 2552
Halo: Reach Datapads- 2552
Halo Wars Timeline (Black Boxes)- 2525-2552
Halo 5: Guardians Mission Intel - 2558
Marketing Campaigns/ ARGs
#HUNTtheTRUTH Season 2 - (2558)
ONI Section 3 (I'll be buggered if I could say I get half of this. It's... Weird)
Halo Board/ Tabletop Games:
Halo Wars RISK
Halo: Fleet Battles - The Fall of Reach
UPCOMING- these will be added into categories as they are released:
Unnamed Spielberg project
Halo Wars 2 (2016)
Halo Mythos Guide (Late 2016)
Halo: The Fall of Reach graphic novel redux - March 2016
Halo: Tales from slipspace (Fall 2016)
As-of-yet unnamed Halo novel listed on Amazon (???)
If I have missed anything, just say, so I can add it.
Useful Links archive- a fairly good list of reliable Halo lore sources and links for online resources
Cheat Sheet - A more complete list that includes campaign/ promotional materials
Edited by BaconShelf, 23 March 2016 - 12:41 PM.