I am trying to create a customized editor window. Inside the editor I want to put a canvas for drawing some elements, and some properties windows, lists etc. I would like to use the editor window with some splitter elements to add other windows, just to be resizable. I know there isn't splitter object in Unity for that, but I have seen some third party apps for unity that use them. Is this possible? http://imgur.com/2Qd1z
Just create seperate windows. EditorWindows can be docked like any other window in Unity. That way the user can arrange everything like he want.
You can of cource create your own "splitter" GUI control, but it depends on what kind of GUI you're using. GUILayout would be difficult since everything is autopositioned. To create your own splitter all you should need is the Event class to process mouseevents.
If you use GUILayout the best thing would be to out each "area" into it's own GUILayout.BeginArea - block and do the positioning yourself.