here's something else that sparked my interest
It is the Halo 3 Forge....similiar to a map editor. I say similiar because i have read that you can change spawns for guns/vehicles/etc as well as other aspects of the game but you cant change the makeup of the map itself
http://www.onthexbox.com/news1179.html here is an article I was reading.
---- Edit: One individual has said that Forge is similiar to the Crackdown Keys to the City idea (except multiplayer)
I also looked at the picture on the article and if those vehicles are all legit then this game is going to drive me nuts