Bungie showed a two-minute video of Halo 3's campaign at E3 2007. Bungie.net has three sizes of WMV variants, and HBO as usual has (or rather will have) WMV and QT versions.

HBO is tagging this as a spoiler, but we've been told the video itself contains no story spoilers.

UPDATE: You can now download the Halo 3 E3 2007 trailer in three sizes in QuickTime.

UPDATE: We're providing a local mirror of the 107Mb, 720p QuickTime file transcoded by Film Oasis and passed on to us by Louis Wu at HBO.