Halo: Custom Edition
From Halopedia, the Halo wiki
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. However, due to the large size of the Halo Custom Edition installation file (and long download times for dial-up users), Halo Custom Edition rarely has more than 200 players on at once.
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.
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There are also 'cheats' for halo CE, called Developer Mode, more commonly called devmode, or dev. It is used by map designers to test out maps they make. It can also be used to cheat in single-player maps that can be downloaded. However they can only be used in single-player maps because when dev is activated, the host can only start a server through LAN. The only way for it to work in multiplayer is to download a small app which allows you to enable or disable devmode without restarting the game, thus allowing you to enable it within a multiplayer game. Even then most of the cheats can only be used by the host.