There are a lot of programs you might need for mod making, but for level making there are very few. Here is a list:
Jed=From the downloads section. You can do everything to make levels in JED, although it does have other uses. Make the arcitecture and then texture your level.
Extras:
A paint program=Anything besides MS Paint. Use it to make new mats with the help of MatMaster. Mats can then be used for the walls and stuff in your level.
COGs= Cog is the JK/MotS scripting language. If you can write 'em, you can do some pretty cool extra features with levels.
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"I am Murray, the all powerful demonic skull, BWAHAHAHAHAHA!"-Murray
Viva la Monkey Island!
"I am Murray, the all powerful demonic skull, BWAHAHAHAHAHA!"-Murray
Viva la Monkey Island!