Gearbox is readying a new version of Halo called Halo Custom Edition, the developer has announced, giving the game's many fans the chance to create mods and other custom content using the v1.04 codebase of the game as a template.
Although reports on this seem a bit confused, the basic idea is that Gearbox will release a new version of Halo in two parts. The first, Halo Custom Edition, will be a 170MB-or-thereabouts executable that resides in its own separate directory and includes lots of new little features and improvements, including adjusted net code and faster performance from pixel shaders. The second, the Halo Editing Kit (HEK), will be about 250MB and consists of the tools, tutorials and artwork needed to create custom content for the PC version of Halo.
The idea behind all this seems to be that if you want to play around with custom content created by other gamers, but don't actually want to make any yourself, you can just download Halo Custom Edition, whereas mod-making types can go for the extra 250MB worth of HEK and get modding.
In case you're wondering, it seems from interviews with Gearbox staff that the separate Custom Edition idea originally came from Microsoft and Bungie, who didn't want the extra hassle of having to support it themselves. As such, the Halo Custom Edition will work completely apart from Halo: Combat Evolved as released on the PC - having not been through Microsoft or Bungie's testing process - but will require an original CD key in order to use. Furthermore, it will be strictly multiplayer (due in part to the size of the single-player maps), and should be released "very, very soon".
For a FAQ document with lots of answers to the obvious questions, this is the link you want. We'll let you know as soon as you can download it.