Looking for a good tutorial on extending the Unity Editor

I am searching for a nice tutorial explaining how to extend the editing capabilities of Unity. In short, I am trying to create a visual scripting extension similar to PlayMaker, and am looking for the right point to start with the editing GUI.

I found this site to be helpfull in the beginning when learning to use the Unity editor:

And make sure you use SerializedProperty variables in the editor script. They are little confusing in the beginning (at least it was for me), but they will make things easier for you in the long run.

This was my first introduction to Editor Scripting (to meshes at the same time!) : http://catlikecoding.com/unity/tutorials/star/

Then I saw this Unite video : http://video.unity3d.com/video/3699926/unite-11-intro-to-editor

Am sure you have the Unity link : Unity - Manual: Extending the Editor with IMGUI

CodeMasterMikes link looks good =]

Unity Editor Extension Tutorial : http://anchan828.github.io/editor-manual/web/index.html