Halo: Custom Edition
From Halopedia, the Halo wiki
Summary
Halo: Custom Edition is a pure free-standing multiplayer add-on for Halo PC. It's free-standing in the sense that you don't need to have Halo PC installed to play the game, however you need the cd-key from the retail game. The point of Halo: Custom Edition is to allow modders to create their own content such as new maps and mods which will hopefully result in a more varied game experience and extend the lifetime of Halo.
Along with the ability to mod the game Gearbox Software also fixed a number of bugs and graphical issues that can still be found in Halo PC.
