As far as I know, the only modeling programs that you can use to create custom content for CE are 3DS Max along with the Blitzkreig plugin or GMax along with the Chimp plugin. The only problem is that 3DS Max, the program that CE was meant to be used with, costs way more than anyone in their right mind would want to spend in order to create content that they were giving out for free. The only other program would then be Gmax which from what I can understand is badly crippled compared to 3DS Max in that you cannot do any animation which is necessary for custom vehicles and weapons, I would assume, for making all of the movements (i.e. reloading a custom weapon, vehicle animations, etc.). I also believe it has become abandoned.
Are there no other options besides these two programs? I know there are other free modeling programs such as Blender, which I use and like, but I don't think there's any way to use it in conjuntion with Halo CE. If not, I guess that means GMax is the only option I have left because my 3DS Max 8 trial expired a while ago. Ugh

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Out of curiosity, has anyone actually purchased a registered version of 3DS Max? Most people here seem to use that, but $3000 seems very pricy for the average person. Maybe you're all rich . . .