Stuff you don't want to miss
Marathon Cheater - An oldy but a goody. It edits your saved games.
Plus Key Saves Anywhere - A neat hack that will allow you to save your game anywhere, by simply pressing the "+" key
Ultimate Hack - A totally different game running on the Marathon Engine. New Sounds, shapes, & maps. Still in development, only three levels are made so far. One neat thing is it changes the waste barrels into explodable waste barrels like DOOM did.