I just jumped right in the deep end with the vortxGE Scene Editor called vortxDEIT, this follows my old editor written in Nokia Qt using C++. But I really wanted to make a native Mac OS X app (Cocoa) for scene editing as this is my main platform these days.
So far it displays all scene node objects and the main Game View.
Left : The Scenes TreeView
Top Middle : Game View
Bottom Middle : Properties
Top Right : Asset Viewer
Bottom Right : Asset TreeView
The interface may change a lot from this design, my aim is to get something up and running fast. Later I'll redesign or even remake the app.