Programs used to make mods?

[Fiddlestyx]

What programs do you use to make mods? I would like to try, but so far all I can do is edit the cfg files.

[hoojiwana]

Blender is a very capable 3D modelling program, and best of all it's free.

You'll also need to make textures with something, and after that use the free version of Unity to make your part into a full game object using the 0.18 Part Tools plugin you can download here.