Macsrule123 has a series of various Marathon movies on his page:

http://www.youtube.com/user/Macsrule123/videos?view=0

Chimera was a total conversion for the Myth II engine made by community members. It turned out so well, it got included in distributions of the Myth Codex.

GelospingmangoMan has Myth walkthroughs, with and without commentary, for the campaign scenarios of Myth 1, Myth 2, and Chimera.

Myth: The Fallen Lords w/o commentary
http://www.youtube.com/playlist?list=PLF5E232DFE2FEE606

QuentaSilmarillion has collected all of the narration sequences that precede each level of Bungie's Myth games. Check out both playlists here:

Myth: The Fallen Lord Narrations
http://www.youtube.com/playlist?list=PLFB527A907952EE93

Myth 2: Soulblighter Narrations
http://www.youtube.com/playlist?list=PLC0AEF07BD94BD594

TirecesIX has quite a collection of Myth-related videos on YouTube. Check out his two playlists:

Myth Videos
http://www.youtube.com/playlist?list=PLAD16E416BA2B1BFC

Even More Myth Videos
http://www.youtube.com/playlist?list=PL70C2CC444AC3CF5C

Tanti1675 has produced a Let's Play video series for both Myth: The Fallen Lords and Myth 2: Soulblighter.

Check out his page for the rest of the videos.

http://www.youtube.com/user/Tanti1675/videos?view=0

DrTheoreticalDonuts on YouTube has done a Let's Play series on Bungie's first 3D first person shooter, Pathways Into Darkness. Find out how a simple human (?) can survive a fall from a crashing airplane, why there are dead German soldiers inside a lost pyramid in the Yucatan, and how to plan an alien musical instrument. Oh, yeah, and nuclear weapons, sleeping gods, and alien ambassadors. Marathon came before Halo, but before Marathon, came Pathways.

For the whole series, check out DrTheoreticalDonuts's playlist:

xxxkeyesxxx started a Marathon 2 walkthrough. Check out his list of videos for the other parts:

http://www.youtube.com/watch?v=Uz34Tdc3nuk&list=UUcj8y3o3ZYmri341dvUf_Xw...

YouTube user KnuckleTurtle also started a Marathon Let's Play; so far it has six parts (from Arrival to Couch Fishing).

Check out KnuckleTurtle's play list for the other videos in the series:

http://www.youtube.com/playlist?list=PLA810F88F20F4C412

Check out Stratafyre's playlist for the complete series.

http://www.youtube.com/watch?v=hsACbWHLJxE&list=PLavZ2dD1TvAGqZ31Eu1WuYv...

ViDoc about Brutes in Halo 3.

DeinOlsen gets out of the Halo 3 level Tsavo Highway using a Warthog.

DevinOlsen has uploaded his classic Halo 3 trick video to YouTube.

DevinOlsen, back again confirming for YouTube that the Food Nipple Grunt easter egg is still in Halo Anniversary.

DevinOlsen confirms that the old Myth easter egg in Halo-- that a certain point outside the control room will play the track "Siege of Madrigal" still exists in the Anniversary edition of the game.