Season eight of the Master Chief Collection just launched, and along with a set of new cosmetics that imagine Spartans from alternate universes comes a set of mod tools that will really let you change Master Chief's look.
Just like how Halo: Combat Evolved received mod tools last summer, now those same tools have arrived for both Halo 2 and Halo 3. This includes world editors, script editors, tag editors, a development build, and more.
First is Guerilla, an app that lets you modify game content files called "tags." This includes enemy AI, textures, animations, models, and much, much more. Second is Sapien, a world/level editor that lets you populate in-game spaces with stuff like vehicles, enemies, and weapons.
Tool is a very basic command-line-only editor, but it's extremely versatile provided you know some coding language. It allows you to quickly make very specific changes without needing to mess around with menus or screens.
And finally, there's Standalone. Basically a development build of the game that's designed to run directly off of tag files instead of cache files so you can get to testing without having to compile everything. It's meant for advanced modifications, so don't go messing about in Standalone unless you really know what you're doing.
343 Industries has also helpfully provided zip files of the level scripts and all tag files for the campaign and multiplayer levels. Note that all tools are provided as-is, with 343 providing minimal support after the fact (they did update Halo 1's mod tools with Season 8, but that was mostly just a name change).
Related Forum: PC Gaming Forum