The developer of the PC version of Halo is about to release some massive updates for the game -- including editing tools, technical updates, and more.
There are two parts to the new software. One part is a new executable and set of data called "Halo Custom Edition" (haloCE.exe) which will reside in its own separate directory. This new executable includes lots of new features, improvements, and capabilities. Things included in the new executable are "Fast Shaders" (improves performance up to 60% on pixel shader hardware), improved network code (reduces the incidence of player "warping"), and lots of new features that provide options for players to improve their multiplayer games.
Perhaps, most importantly, however, the new executable allows end users to play a potentially endless volume of new content created with the Halo Editing Kit. Halo Custom Edition (HaloCE.exe) and the supporting data is about 200mb and the HEK is about 250mb.
News source: Gamespy