Marathon saved games can be edited with this. Note that the version 2.5 is for the real game, 1.0 is for the Demo. This works great and never hangs. You can give yourself more ammo, invisibility etc.
2.  
MapEdit Pro 1.1 Alpha
By Bernie Freidin
This is a program for editing the levels of Marathon. Currently it is not a finished product and you as of now you canΓÇÖt do much with it. Also, it crashes often.
3.  
Texture Viewer
I do now know who has written this. All it does is that it shows all the textures from the Shapes file.
4.  
Marathon Shape Installer 0.9b1
By Kevin Bell
This shows the textures, too, but you can paste your own textures to replace the original ones.
5.  
Marathon Physics Model Editor 1.1
By Michael Hanson
This is a great tool with which you can change many aspects of the game You can make the pistol fire missiles at a rapid rate, you can remove the gravity, you can make Bobs hostile… This is almost a must if you play Marathon.
 6.
Marathon Kit
By Sean Hollick
This is not actually a program; it is a collection of PICT files that contain a map for every level in Marathon. A good aid for beginners.
 7.
Marathon v1.0==>v1.1 Updater
Bungie has released an updater that fixes a number of bugs (see Bugs and Conflicts). Besides those already mentioned, Bungie says these have been fixed:
ΓÇó Some Ram Doubler conflicts were identified and fixed.
ΓÇó Sometimes changing bit depths or monitors didnΓÇÖt take effect until quitting and relaunching Marathon; this has been fixed.
ΓÇó On 12ΓÇ¥ monitors, 640x400 PowerBooks, etc., the Setup Keys dialog no longer appears with the ΓÇ£OKΓÇ¥ button off the screen.
Some enhancements:
ΓÇó Millions of colors is now an option; check out the smooth lighting effects if youΓÇÖve got the hardware (and about 15M RAM).
ΓÇó Slightly faster rendering code.
ΓÇó All keyboard controls now work while mouse control is active (turn left/right, look up/down, recenter, etc.)
Many people have complained about the new mouse control, which was changed because of the bug in it. You might need to change the speed of your mouse to a slower setting in order to make the game more playable.