Halo 3's forge mode - the game's editing suite - is in for a serious revamp when Halo: Reach is released in September.
As Mike saw earlier today at Comic-Con, Halo: Reach has replaced Forge with Forge World, a vastly improved editing tool that not only lets players craft much larger maps, but also have a much more detailed method of interacting with them.
As an example, there will be multiplayer maps shipping on Reach's disc that were created using only Forge World. And that's from a dev team under crunch to get a game shipped. When the community get their hands on it, I expect amazing things to happen.