obj2mc
Converts a 3d model from .obj format into the json form recognized by Minecraft. Usage:
python obj2mc.py myobject.obj [growthfactor]
growthfactor is an optional argument that increases or decreases the overall size of the model. For example, growthfactor of 2 will double all three dimensions, and 0.5 will halve them.
Note that for high-resolution models, you might want to lower the poly count using some other tool first, at least if you want Minecraft to be able to render it without crashing.
At the moment, only the format used in 16w06b is supported, not later versions.