If you don't like how I drive, then stay off the sidewalk!
- ʟ₳ẔӬЯ
The basic tools used for modding Halo PC are HHT (Halo Hacking Tools), HMT (Halo Map Tools), SparkEdit, and more recently Eschaton. The whole list is here: http://www.modhalo.net/index.php?/topic/28-halo-pc-tools-list /
Also, I think you would benefit from joining that site I linked to. We have sections for normal Halo PC, and some for Custom Edition.
[Edited on 03.31.2011 3:43 PM PDT]