In ODST his character yells out "Gorramit!" occasionally, another Firefly reference.
Katee Sackhoff, the actress who portrays Kara Thrace in the 2003 revival of Battlestar Galactica, voiced a marine in Halo 3. You can hear her yelling "Frak!" if she gets hits.
Aluminum Christmas Trees: There really is a "Master Chief" rank in the US military - Master Chief Petty Officer is a naval rank, and yes, the correct way to address one is "Master Chief". Quite understandably, when a suspect of such a rank once showed up on NCIS, surprise and confusion ensued for some.
Cash Cow Franchise: Four main games (fifth one in the works), various books and comics, the requisite action figures, an anime series, five Gaiden Games (including two on mobile), and three live-action serials, not to mention various promotional tie-ins.
God Never Said That: Before Halo Wars, Halo fanon had claimed that the armor worn by Arbiters was the same exact suit that it had been through history, the successor receiving it after his predecessor had died (which is implied, but not said, in Halo 2, with the Prophets saying that the bodies were all in the mausoleum, and the armor's systems being outdated). However, the Arbiter of Halo WarsRipa 'Moramee was killed and pushed down a chasm before his body was destroyed in a Earth-Shattering Kaboom. Henceforth, that meant the armor had been destroyed with it. Fans were annoyed, but it was later pointed out that since the Arbiter is intended to be sent on suicide missions, there would have been many cases where the armor was destroyed and/or impossible to recover. Such as falling to the core of a gas giant, like Thel 'Vadamee was about to.
Hey, It's That Voice!: Early-rising fans of classic rock in Chicago will recognize Master Chief's voice as that of Steve Downes, the 5AM-10AM DJ on 97.1FM. Bungie was based in Chicago before being bought by Microsoft.
Jossed: Halo 3's entire storyline, specifically The Forerunners' relationship with the Flood, the Forerunner's relationship with humanity, and the possible (lack of, as it turned out) ulterior motives the Prophet of Truth had for activating Halo.
The Film of the Game was going to be directed by Neill Blomkamp, the director of District 9.
Of course, District 9 was only made because Blomkamp couldn't do the Halo movie. If there had been a Halo movie, we might very be lamenting the opposite situation.
The Wiki Rule: Two, Halopedia, an independent wiki and Halo Nation, which is hosted by Wikia. The two were previously one wiki, but split apart after a dispute about the new Oasis skin for all Wikia websites. The two share many of the same administrators but have a tense relationship otherwise. There was also a third, Halo Wiki.net, which focused primarily on multiplayer, but is since mostly defunct.