Here's a list of programs you can use to create add-ons
Weapons, vehicles, player models, and bricks:
3D modeling
• Blender
Portify's DTS/DSQ import/export plugins
• Milkshape 3D
Texture creation (optional)
• Paint.NET
• Adobe Photoshop
• GIMP
Scripting
• Notepad
• Notepad++
Sounds
• Audacity
Skyboxes, water, ground:
Textures
• Paint.NET
• Adobe Photoshop
• GIMP
Scripting
• Notepad
Prints:
Textures
• Paint.NET
• Adobe Photoshop
• GIMP