SparkEdit
From Halo Mod Maps Wiki
Introduction
SparkEdit is the tool we use edit maps. It has a duplicate function to add vehicles (if that vehicle is on the map) and scenery and a teleporter creation feature to create teleporters.
Why we use it
We use SparkEdit because it is a very powerful and an easy to use tool for modding our maps. The following is a quote from Download.com. SparkEdit allows you to explore Halo maps from a variety of viewpoints. SparkEdit allows players to view Halo maps from several viewpoints. Do a walkthrough or flythrough of your favorite map to see how it is constructed. This is a great tool for mappers to get ideas and inspiration for their own custom maps.
Credits
This program was created by Grendiac not just for us but for everybody. Download it here