Forge
From Halopedia, the Halo wiki
Forge is the Map Editor in Halo 3, confirmed in the EGM Scans. You will tweak elements in the map, such as removing crates or adding Fusion cores. You will also be able to tweak "minor" weapon (and possibly vehicle) statistics, such as "Ammo Capacity."
You take the role of a monitor (Such as 343 Guilty Spark). You can then travel throughout the map placing, picking up, moving, and deleting objects on the map. You can also add objects as well, but you only have a certain amount before you can't add anything else.
In a sense this isn't a map editor that you would have expected. You're not actually building and designing maps, you're manipulating the existing maps to a certain way you would like.
Items that can be tampered with include crates and barricades, as well as weapon, equipment and vehicle spawns. It has been stated that the UNSC Elephant will not be among the objects due to its size and specific purpose on maps. Throughout the time you're editing you can press up on the D-Pad and change to your multiplayer Spartan/Elite model and play.
Once you're done with the editing you can start a party over System Link or Xbox Live and see how it works. You can build crate towers, grab a vehicle underneath a teammate, or have a friend jump on a crate and move it throughout the map.
Forge will keep track of stats and kills. Unfortunatly, once you're done, you might not be able to save the creation you have made. You will probably just have to rebuild it the next time. This information has yet to be confirmed, so there is a chance of being able to save, but information about playing multiplayer in forge indicates that it may not be able to.
