I need a free (preferably open source) clothing design program.
What I found:
For example, the section "Creating a garment with Optitex and Blender: Constructing a Boudicca dress" (of Thomas Makryniotis. 2015. 3D Fashion Design: Technique, Design and Visualization. Pavilion Books.) mentions Blender, which is a free open source 3D modeling program.
cf. my related Blender Stack Exchange question: Can Blender be used to design 2D clothing patterns?
Inkscape is the best free open source 2D design program, but it is not specifically intended for creating clothing patterns, although one could certainly use it for that purpose.