Marathon's Story is the ULTIMATE source of information on the Marathon trilogy, and more recently, Halo. Even though the Marathon trilogy is coming up for 5 years old, people are still discovering new things in the story. The information on this site totals something like 7 Gb. Updated almost every day, this site is a MUST VISIT for the self-respecting Bungie fanatic.
Formerly the Core, Rampancy.net is the best source for up-to-date information on Halo. They also cover other Bungie games (Marathon/Marathon Open Source, Myth, Oni), although its main focus is Halo. The best feature of Rampancy.net is the discussion forum, where fans can exchange ideas and information and speculate on various features of Halo. Another must visit.
HBO is the Halo section of the galaxy-spanning Bungie.org family. Similar to Rampancy.net (although without the news coverage of games other than Halo), the news is updated freqently, and the site also sports a discussion forum.
Bungie.org is huge. They cover all Bungie games: Pathways into Darkness, Marathon, Myth, Oni, and Halo (phew). There is also a section called BS (Bungie Sightings). The description, taken from the site:
We are Bungie Sightings, or BS for short. Yes, we have a sense of humor! Unlike other Bungie related sites, our focus is only on two things, visuals and community. We are looking for any pictures you may have that depict a Bungie product in an interesting situation. Do you have Oni stickers plastered on your desk? Well, that might be interesting to you, but maybe not to most people. Got a picture of yourself wearing a Bungie "Don't make us kick your ass!" T-Shirt while standing in front of the Grand Canyon? Now that's more like it! How about your grandmother holding up a Marathon Trilogy box? Cool! Can you wave an Oni Door Poster from the top of the Eiffel Tower? Awesome! Take a picture and send it on in...
Inside Mac Games
As the name would suggest, Inside Mac Games is a Macintosh gaming news site (what a surprise!). The news is updated every weekday. They also publish a monthly magazine.
Clan Plaid's Townhall website is dedicated to Myth The Fallen Lords and Myth II Soulblighter news. Unfortunately, the news seems to be updated rather at random. It sports several discussion forums which are quite active, though.
Absolute Halo is...a website. Yep, its definitely a website. Run by some fellow called "Hiro"...maybe you've heard of him? Its not too shabby...worth a visit.