Halo: Custom Edition was a Multiplayer expansion for Halo: Combat Evolved, and was released in 2004 by Gearbox Studios. It requires the original game to function, and only works for the PC.
The expansion was released in 2004, due to the lack of a Multiplayer function in Halo: Combat Evolved. The software allows for map creation, modding and creation of much more models. The release of this spawned many modding communities, helping the popularity of Halo.