Unity Part Tools
From Kerbal Space Program Wiki
The newest Unity Part Tools for KSP (currently version 1.12.x) can be found in this forum thread. There is no longer a working download link at kerbalspaceprogram.com.
The tool is used to create new parts for Kerbal Space Program and requires you to have a copy of Unity 2019.4.18f1 LTS installed.
Additional modders' notes are in other threads in the forum, notably here for KSP 1.12.x.