Unity Editor Tools - Create Better Games Faster! - Hungry Ghost by Nanaimo Labs
Runtime Unity Editor - A fork from RuntimeUnityEditor which makes it compatible with Unity 4.1
Quickstart: Install & configure Visual Studio Tools for Unity | Microsoft Learn
Custom Editor Coding - Tutorial One - Custom Foldouts - Unity Forum
If you make Unity Editor tools, try the new Editor Foundations Design System: This covers helpful guidance, best practices, and other resources to help users build a better Editor UI. : r/Unity3D