PartTools
A package for Unity 2019.4.18f that assists in creating .mu files and IVA layouts. The TextMeshPro package can be found here.
Starilex NightStar
A better version of PartTools for IVA development. The official PartTools has a bug in locales where comma is used as a decimal separator; this fixes that and adds several more tools that are useful for placing props.
VSCode
Mostly a text editor, but can also be configured to compile C#. Works great for editing .cfg files with this extension