Halo Zero Level Editor
From Halopedia, the Halo wiki
Summary
The Halo Zero Level Development Tool is a level editor for Halo Zero that allows you to make modifications that you can play without any parent-application. If you want to find or share modifications you should look at the Halo Zero Modifications website
The current version of the Halo Zero Editor is 1.0.1 and has been available since 2006. The dev tool will most likely not be updated anymore since Dobermann lost his cd-key to the program he made it in.
The current version of the level editor is quite unstable and you have to back the levels up quite often.
Mods
The Level Editor allows players, mainly the members of the Halo Zero Community, to mod maps.